Hashimoto's thyroiditis

Hashimoto's thyroiditis, also known as chronic lymphocytic thyroiditis, Hashimoto's disease and autoimmune thyroiditis, is an autoimmune disease in which the thyroid gland is gradually destroyed.<ref name="AR">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="niddk2024">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Early on, symptoms may not be noticed.<ref name=NIH2014>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Over time, the thyroid may enlarge, forming a painless goiter.<ref name=NIH2014/> Most people eventually develop hypothyroidism with accompanying weight gain, fatigue, constipation, hair loss, and general pains.<ref name="niddk2024"/> After many years the thyroid typically shrinks in size.<ref name="niddk2024"/> Potential complications include thyroid lymphoma.<ref name=Nou2015>Template:Cite journal</ref> Further complications of hypothyroidism can include high cholesterol, heart disease, heart failure, high blood pressure, myxedema, and potential problems in pregnancy.<ref name="niddk2024"/>

Hashimoto's thyroiditis is thought to be due to a combination of genetic and environmental factors.<ref name="Ramos-Levi2023" /><ref name="Winther-2020">Template:Cite journal</ref> Risk factors include a family history of the condition and having another autoimmune disease.<ref name=NIH2014/> Diagnosis is confirmed with blood tests for TSH, Thyroxine (T₄), antithyroid autoantibodies, and ultrasound.<ref name=NIH2014/> Other conditions that can produce similar symptoms include Graves' disease and nontoxic nodular goiter.<ref name=Ak2000>Template:Cite book</ref>

Hashimoto's is typically not treated unless there is hypothyroidism, or the presence of a goiter, when it may be treated with levothyroxine.<ref name=Ak2000/><ref name=NIH2014/> Those affected should avoid eating large amounts of iodine; however, sufficient iodine is required especially during pregnancy.<ref name=NIH2014/> Surgery is rarely required to treat the goiter.<ref name=Ak2000/>

Hashimoto's thyroiditis has a global prevalence of 7.5%, and varies greatly by region.<ref name="Hu"/> The highest rate is in Africa, and the lowest in Asia.<ref name="Hu"/> In the US white people are affected more often than black. It is more common in low to middle income groups. Females are more susceptible with a 17.5% rate of prevalence compared to 6% in males.<ref name="Hu"/> It is the most common cause of hypothyroidism in developed countries.<ref name="Mincer2022">Template:Cite book</ref> It typically begins between the ages of 30 and 50.<ref name=NIH2014/><ref name=Hir2013>Template:Cite journal</ref> Rates of the disease have increased.<ref name="Hu"/> It was first described by the Japanese physician Hakaru Hashimoto in 1912.<ref>Template:Cite book</ref> Studies in 1956 discovered that it was an autoimmune disorder.<ref>Template:Cite journal</ref> Template:TOC limit

Signs and symptoms

File:Signs and symptoms of hypothyroidism.png

Systemic manifestations of hypothyroidism

Signs

File:Hypothyroidism.jpg

Depiction of a goiter

In the early stages of autoimmune thyroiditis, patients may have normal thyroid hormone levels and no goiter or a small one.<ref name="Ramos-Levi2023" /> Enlargement of the thyroid is due to lymphocytic infiltration and fibrosis.<ref name="Klubo-Gwiezdzinska-2022">Template:Cite journal</ref> Early on, thyroid autoantibodies in the blood may be the only indication of Hashimoto’s disease.<ref name="Ramos-Levi2023" /> They are thought to be the secondary products of the T cell-mediated destruction of the gland.<ref name="Ramos-Levi2023" />

As lymphocytic infiltration progresses, patients may exhibit signs of hypothyroidism in multiple bodily systems, including, but not limited to, a larger goiter, weight gain, cold intolerance, fatigue, myxedema, constipation, menstrual disturbances, pale or dry skin, and dry, brittle hair, depression, and ataxia.<ref name="Singh2020">Template:Cite book</ref><ref name="Mincer2022" /> Extended thyroid hormone deficiency may lead to muscle fibre changes, resulting in muscle weakness, muscle pain, stiffness, and rarely, pseudohypertrophy.<ref name="Fariduddin-2024">Template:Cite book</ref> Patients with goiters who have had autoimmune thyroiditis for many years might see their goiter shrink in the later stages of the disease due to destruction of the thyroid.<ref name="niddk2024" /> Graves disease may occur before or after the development of autoimmune thyroiditis.<ref name="Weetman2021">Template:Cite book</ref>

While rare, more serious complications of the hypothyroidism resulting from autoimmune thyroiditis are pericardial effusion, pleural effusion, both of which require further medical attention, and myxedema coma, which is an endocrine emergency.<ref name="Mincer2022" />

Symptoms

Many symptoms are attributed to the development of Hashimoto's thyroiditis. Symptoms can include: fatigue, weight gain, pale or puffy face, feeling cold, joint and muscle pain, constipation, dry and thinning hair, heavy menstrual flow or irregular periods, depression, a slowed heart rate, problems getting pregnant, miscarriages,<ref>Template:Cite news</ref> and myopathy.<ref name="Fariduddin-2024" /> Some patients in the early stage of the disease may experience symptoms of hyperthyroidism due to the release of thyroid hormones from intermittent thyroid destruction<ref name="Mincer2022" /><ref name="Dyrka-2024">Template:Cite journal</ref> (also called "destructive thyrotoxicosis").<ref name="Ramos-Levi2023" /> In non-medical settings, the term "flare" is used to refer to a sudden exacerbation of symptoms, whether hyper or hypo.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

While most symptoms are attributed to hypothyroidism, similar symptoms are observed in Hashimoto's patients with normal thyroid hormone levels.<ref name=":6">Template:Cite book</ref><ref name="Hegedüs-2022" /><ref name="Klubo-Gwiezdzinska-2022" /> According to one study, these symptoms may include lower quality of life, and issues of the "digestive system (abdominal distension, constipation and diarrhea), endocrine system (chilliness, gain weight and facial edema), neuropsychiatric system (forgetfulness, anxiety, depressed, fatigue, insomnia, irritability, and indifferent [sic]) and mucocutaneous system (dry skin, pruritus, and hair loss)."<ref name="Li-2024">Template:Cite journal</ref>

Causes

The causes of Hashimoto's thyroiditis are complex. Around 80% of the risk of developing an autoimmune thyroid disorder is due to genetic factors, while the remaining 20% is related to environmental factors (such as iodine, drugs, infection, stress, radiation).<ref name="Casto-2021">Template:Cite journal</ref>

Genetics

Thyroid autoimmunity can be familial.<ref name="Dayan96">Template:Cite journal</ref> Many patients report a family history of autoimmune thyroiditis or Graves' disease.<ref name="Singh2020"/> The strong genetic component is borne out in studies on monozygotic twins,<ref name="Mincer2022" /> with a concordance of 38–55%, with an even higher concordance of circulating thyroid antibodies not in relation to clinical presentation (up to 80% in monozygotic twins). Neither result was seen to a similar degree in dizygotic twins, offering strong favour for high genetic etiology.<ref name="Chistiakov-2005" />

The genes implicated vary in different ethnic groups<ref name="Jacobson-2008" /> and the impact of these genes on the disease differs significantly among people from different ethnic groups. A gene that has a large effect in one ethnic group's risk of developing Hashimoto's thyroiditis might have a much smaller effect in another ethnic group.<ref name="Chistiakov-2005" />

The incidence of autoimmune thyroid disorders is increased in people with chromosomal disorders, including Turner, Down, and Klinefelter syndromes.<ref name="Casto-2021" />

HLA genes

The first gene locus associated with autoimmune thyroid disease was the major histocompatibility complex (MHC) region on chromosome 6p21. It encodes human leukocyte antigens (HLAs). Specific HLA alleles have a higher affinity to auto-antigenic thyroidal peptides and can contribute to autoimmune thyroid disease development. Specifically, in Hashimoto's disease, aberrant expression of HLA II on thyrocytes has been demonstrated. They can present thyroid autoantigens and initiate autoimmune thyroid disease.<ref name="Jacobson-2008">Template:Cite journal</ref> Susceptibility alleles are not consistent in Hashimoto's disease. In Caucasians, various alleles are reported to be associated with the disease, including DR3, DR5, and DQ7.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

CTLA-4 genes

CTLA-4 is the second major immune-regulatory gene related to autoimmune thyroid disease. CTLA-4 gene polymorphisms may contribute to the reduced inhibition of T-cell proliferation and increase susceptibility to autoimmune response.<ref>Template:Cite journal</ref> CTLA-4 is a major thyroid autoantibody susceptibility gene. A linkage of the CTLA-4 region to the presence of thyroid autoantibodies was demonstrated by a whole-genome linkage analysis.<ref>Template:Cite journal</ref> CTLA-4 was confirmed as the main locus for thyroid autoantibodies.<ref>Template:Cite journal</ref>

PTPN22 gene

PTPN22 is the most recently identified immune-regulatory gene associated with autoimmune thyroid disease. It is located on chromosome 1p13 and expressed in lymphocytes. It acts as a negative regulator of T-cell activation. Mutation in this gene is a risk factor for many autoimmune diseases. Weaker T-cell signaling may lead to impaired thymic deletion of autoreactive T cells, and increased PTPN22 function may result in inhibition of regulatory T cells, which protect against autoimmunity.<ref>Template:Cite journal</ref>

Immune-related genes

IFN-γ promotes cell-mediated cytotoxicity against thyroid mutations causing increased production of IFN-γ were associated with the severity of hypothyroidism.<ref>Template:Cite journal</ref> Severe hypothyroidism is associated with mutations leading to lower production of IL-4 (Th2 cytokine suppressing cell-mediated autoimmunity),<ref>Template:Cite journal</ref> lower secretion of TGF-β (inhibitor of cytokine production),<ref>Template:Cite journal</ref> and mutations of FOXP3, an essential regulatory factor for the regulatory T cells (Tregs) development.<ref>Template:Cite journal</ref> Development of Hashimoto's disease was associated with mutation of the gene for TNF-α (stimulator of the IFN-γ production), causing its higher concentration.<ref>Template:Cite journal</ref>

Existential (endogenous environmental)

Sex

Study of healthy Danish twins divided to three groups (monozygotic and dizygotic same sex, and opposite sex twin pairs) estimated that genetic contribution to thyroid peroxidase antibodies susceptibility was 61% in males and 72% in females, and contribution to thyroglobulin antibodies susceptibility was 39% in males and 75% in females.<ref>Template:Cite journal</ref>

The high female predominance in thyroid autoimmunity may be associated with the X chromosome. It contains sex and immune-related genes responsible for immune tolerance.<ref>Template:Cite journal</ref> A higher incidence of thyroid autoimmunity was reported in patients with a higher rate of X-chromosome monosomy in peripheral white blood cells.<ref>Template:Cite journal</ref> Another potential mechanism might be skewed X-chromosome inactivation.<ref name="Ramos-Levi2023" />

Pregnancy

In one population study, two or more births were a risk factor for developing autoimmune hypothyroidism in pre-menopausal women.<ref name="Carlé-2014">Template:Cite journal</ref>

Environmental

Medications

Certain medications or drugs have been associated with altering and interfering with thyroid function. There are two main mechanisms of interference:<ref name="Surks-1995" />

• Altering thyroid hormone serum transfer proteins.<ref name="Surks-1995">Template:Cite journal</ref> Estrogen, tamoxifen, heroin, methadone, clofibrate, 5-fluorouracil, mitotane, and perphenazine all increase thyroid binding globulin (TBG) concentration.<ref name="Surks-1995" /> Androgens, anabolic steroids such as danazol, glucocorticoids, and slow release nicotinic acid all decrease TBG concentrations. Furosemide, fenoflenac, mefenamic acid, salicylates, phenytoin, diazepam, sulphonylureas, free fatty acids, and heparin all interfere with thyroid hormone binding to TBG and/or transthyretin.<ref name="Surks-1995" />
• Altering extra-thryoidal metabolism of thyroid hormone. Propylthiouracil, glucocorticoids, propranolol, iondinated contrast agents, amiodarone, and clomipramine all inhibit conversion of T₄ and T₃.<ref name="Surks-1995" /> Phenobarbital, rifampin, phenytoin and carbamazepine all increase hepatic metabolism.<ref name="Surks-1995" /> Finally, cholestryamine, colestipol, aluminium hydroxide, ferrous sulphate, and sucralfate are all drugs that decrease T₄ absorption or enhance excretion.<ref name="Surks-1995" />

Iodine

Both excessive and insufficient iodine intake has been implicated in developing antithyroid antibodies.<ref name=":9" /><ref name=":8">Template:Cite book</ref> Thyroid autoantibodies are found to be more prevalent in geographical areas after increasing iodine levels.<ref name=":8" /> Several mechanisms by which excessive iodine may promote thyroid autoimmunity have been proposed:<ref name=":9" />

• Via thyroglobulin iodination: Iodine exposure leads to higher iodination of thyroglobulin, increasing its immunogenicity<ref name=":9">Template:Cite journal</ref> by creating new iodine-containing epitopes or exposing cryptic epitopes.<ref>Template:Cite journal</ref>
• Via thyrocyte damage: Iodine exposure has been shown to increase the level of reactive oxygen species. They enhance the expression of the intracellular adhesion molecule-1 on the thyrocytes, which could attract the immuno-competent cells into the thyroid gland.<ref name=":9" /> Iodine also promotes thyrocyte apoptosis.<ref name=":9" />
• Via immune cell behaviour: Iodine has an influence on immune cells.<ref name=":9" />

Comorbidities

Comorbid autoimmune diseases are a risk factor for developing Hashimoto's thyroiditis, and the opposite is also true.<ref name=NIH2014/> Another thyroid disease closely associated with Hashimoto's thyroiditis is Graves' disease.<ref name="Weetman2021"/> Autoimmune diseases affecting other organs most commonly associated with Hashimoto's thyroiditis include celiac disease, type 1 diabetes, vitiligo, alopecia,<ref name="Radetti2014">Template:Cite book</ref> Addison disease, Sjogren's syndrome, and rheumatoid arthritis<ref name="Singh2020"/><ref name="Niddk2021">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Autoimmune thyroiditis has also been seen in patients with autoimmune polyendocrine syndromes type 1 and 2.<ref name="Weetman2021"/>

Other

Other environmental factors include selenium deficiency,<ref name="Winther-2020" /> infectious diseases such as hepatitis C, rubella, and possibly Covid-19,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> toxins,<ref name="Ramos-Levi2023" /> dietary factors,<ref name="Weetman2021"/> radiation exposure,<ref name="Ramos-Levi2023" /> and gut dysbiosis.<ref name="Ludgate-2024" />

Mechanism

The pathophysiology of autoimmune thyroiditis is not well understood.<ref name="Ramos-Levi2023">Template:Cite book</ref> However, once the disease is established, its core processes have been observed:

Hashimoto's thyroiditis is a T-lymphocyte mediated attack on the thyroid gland.<ref name="Klubo-Gwiezdzinska-2022" /> T helper 1 cells trigger macrophages and cytotoxic lymphocytes to destroy thyroid follicular cells, while T helper 2 cells stimulate the excessive production of B cells and plasma cells which generate antibodies against the thyroid antigens, leading to thyroiditis.<ref name="ref 4" /> The three major antibodies are: Thyroid peroxidase Antibodies (TPOAb), Thyroglobulin Antibodies (TgAb), and Thyroid stimulating hormone receptor Antibodies (TRAb),<ref name="Dayan96" /> with TPOAb and TgAb being most commonly implicated in Hashimotos.<ref name="Ramos-Levi2023" /> They are hypothesized to develop as a result of thyroid damage, where T-lymphocytes are sensitized to residual thyroid peroxidase and thyroglobulin, rather than as the initial cause of thyroid damage.<ref name="Ramos-Levi2023" /> However, they may exacerbate further thyroid destruction by binding the complement system and triggering apoptosis of thyroid cells.<ref name="Ramos-Levi2023" /> TPO antibody levels may correlate with the degree of lymphocyte infiltration of the thyroid.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Gross morphological changes within the thyroid are seen in the general enlargement, which is far more locally nodular and irregular than more diffuse patterns (such as that of hyperthyroidism). While the capsule is intact and the gland itself is still distinct from surrounding tissue, microscopic examination can provide a more revealing indication of the level of damage.<ref name="Maitra 2014 The Endocrine System">Template:Cite book</ref> Hypothyroidism is caused by replacement of follicular cells with parenchymatous tissue.<ref name="ref 4">Template:Cite journal</ref>

Partial regeneration of the thyroid tissue can occur, but this has not been observed to normalise hormonal levels.<ref>Template:Cite journal</ref><ref name="Ushakov-2024">Template:Cite journal</ref>

Pathology

File:Hashimoto's thyroiditis, HE 3.jpg

Marked lympocytic infiltration (purple areas) of the thyroid gland in a patient with chronic autoimmune thyroiditis

File:Hashimoto thyroiditis -- high mag.jpg

High powered magnification showing lymphocytic infiltration of the thyroid gland in autoimmune thyroiditis

Gross pathology of a thyroid with autoimmune thyroiditis may show an symmetrically enlarged thyroid.<ref name="Ramos-Levi2023" /> It is often paler in color, in comparison to normal thyroid tissue which is reddish-brown.<ref name="Ramos-Levi2023" />

Microscopic examination (histology) will show lymphocytes (including plasma B-cells) diffusely infiltrating the parenchyma.<ref name="Maitra 2014 The Endocrine System" /> The lymphocytes are predominately T-lymphocytes with a representation of both CD4+ and CD8+ cells.<ref name="Ramos-Levi2023" /> The plasma cells are polyclonal, with present germinal centers resembling the structure of a lymph node<ref name="Ramos-Levi2023" /> (also called secondary lymphoid follicles, not to be confused with the normally present colloid-filled follicles that constitute the thyroid).<ref name="Maitra 2014 The Endocrine System" />

In late stages of the disease, the thyroid may be atrophic.<ref name="Mincer2022" /> Colloid-filled follicles shrink and the cuboidal cells that usually line the follicles become Hürthle cells.<ref name="Ramos-Levi2023" /> Fibrous tissue may be found throughout the affected thyroid as well.<ref name="Ramos-Levi2023" /> Severe thyroid atrophy presents often with denser fibrotic bands of collagen that remains within the confines of the thyroid capsule.<ref name="Maitra 2014 The Endocrine System" />

Generally, pathological findings of the thyroid are related to the amount of existing thyroid function — the more infiltration and fibrosis, the less likely a patient will have normal thyroid function.<ref name="Ramos-Levi2023" /> A rare but serious complication is thyroid lymphoma, generally the B-cell type, non-Hodgkin lymphoma.<ref name="Dayan96" />

Diagnosis

File:Hashimoto-Thyreoiditis.JPG

Ultrasound imaging of the thyroid gland (right lobe longitudinal) in a person with Hashimoto thyroiditis

Tests

Physical exam

Physicians will often start by assessing reported symptoms and performing a thorough physical exam, including a neck exam.<ref name="Mincer2022" /> Patients may have a "firm, bumpy, symmetric, painless goiter", however, up to 10% of patients may have an atrophied thyroid.<ref name="Ramos-Levi2023" />

Antithyroid antibodies tests

Tests for antibodies against thyroid peroxidase, thyroglobulin, and thyrotropin receptors can detect autoimmune processes against the thyroid. 90% of hashimoto's patients have elevated levels of thyroid peroxidase antibodies.<ref name="Ramos-Levi2023" /> However, seronegative (without circulating autoantibodies) thyroiditis is also possible.<ref>Template:Cite journal</ref> There may be circulating antibodies before the onset of any symptoms.<ref name="Mincer2022" />

Ultrasound

File:Hashimoto Thyroiditis.jpg

Ultrasound imaging of the thyroid showing Hashimoto's thyroiditis

An ultrasound may be useful in detecting Hashimoto thyroiditis, especially in those with seronegative thyroiditis,<ref name="Klubo-Gwiezdzinska-2022" /> or when patients have normal laboratory values but symptoms of autoimmune thyroiditis.<ref name="Niddk2021" /> Key features detected in the ultrasound of a person with Hashimoto's thyroiditis include "echogenicity, heterogeneity, hypervascularity, and presence of small cysts."<ref name="Klubo-Gwiezdzinska-2022" /> Images obtained with ultrasound can evaluate the size of the thyroid, reveal the presence of nodules, or provide clues to the diagnosis of other thyroid conditions.<ref name="Niddk2021" />

Nuclear medicine

Nuclear imaging showing thyroid uptake can also be helpful in diagnosing thyroid function, particularly differential diagnosis.<ref name="Ramos-Levi2023" />

TSH levels test

Elevated Thyroid-stimulating hormone (TSH) levels may indicate hypothyroidism (underpeforming thyroid).<ref name="Niddk2021"/> Hypothyroidism is a common symptom and potential indication of Hashimoto's disease.<ref name="Ramos-Levi2023" /> As blood levels of thyroid hormones fall due to hypothyroidism, the anterior pituitary gland increases production of TSH, which stimulates increased production of thyroid hormones in the thyroid.<ref name=":6" /> The elevation is usually a marked increase over the normal range.<ref name="Singh2020"/> TSH is the preferred initial test of thyroid function as it has a higher sensitivity to changes in thyroid status than free T₄.<ref>Template:Cite book</ref>

Biotin can cause this test to read "falsely low".<ref name=":6" /> Time of day can affect the results of this test; TSH peaks early in the morning and slumps in the late afternoon to early evening,<ref>Template:Cite journal</ref> with "a variation in TSH by a mean of between 0.95 mIU/mL to 2.0 mIU/mL".<ref>Template:Cite journal</ref> Hypothyroidism is diagnosed more often in samples taken soon after waking.<ref>Template:Cite journal</ref>

T₃ or T₄ levels test

These tests detect levels of two thyroid hormones: Thyroxine (T₄) and Tri-iodothyronine (T₃). Low levels of these hormones (hypothyroidism) may indicate autoimmune damage to the thyroid due to Hashimoto's, while elevated levels may indicate an attack of destructive thyrotoxicosis.<ref name="Ramos-Levi2023" /> Hashimotos with normal levels is possible however.

Free or total levels can be measured. Typically, Free T₄ is the preferred test for hypothyroidism,<ref name="Van Uytfanghe-2023">Template:Cite journal</ref> as Free T₃ immunoassay tests are less reliable at detecting low levels of thyroid hormone,<ref>Template:Cite book</ref> and they are more susceptible to interference.<ref name="Van Uytfanghe-2023" /> Both immunoassay tests of Free T₄ and Free T₃ may overestimate concentrations, particularly at low thyroid hormone levels, which is why results are typically read in conjunction with TSH, a more sensitive measure.<ref name="Welsh-2016">Template:Cite journal</ref> LC-MSMS assays are rarer, but they are "highly specific, sensitive, precise, and can detect hormones found in low concentrations."<ref name="Welsh-2016" />

Muscle Biopsy

Muscle biopsy is not necessary for diagnosis of myopathy due to hypothyroid muscle fibre changes, however it may reveal confirmatory features.<ref name="Fariduddin-2024" />

Treatment

There is no cure for Hashimoto's Thyroiditis.<ref name="Ludgate-2024">Template:Cite journal</ref><ref name="Australia-2023">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> There is currently no known way to stop auto-immune lymphocytes infiltrating the thyroid or to stimulate regeneration of thyroid tissue.<ref name="Ramos-Levi2023" /> However, the condition can be managed.<ref name="Ludgate-2024" /><ref name="Australia-2023" />

File:T4 LT4 LT4 Sodium - T3 LT3 LT3 Sodium.png

Molecular structure of Thyroxine, Levothyroxine, Levothyroxine Sodium, Tri-iodothyronine, Liothyronine, and Liothyronine Sodium.

Managing hormone levels

Hormone Terminology

	Endogenous	Synthetic
T3	Tri-iodothyronine	Liothyronine
T4	Thyroxine	Levothyroxine

Hypothyroidism caused by Hashimoto's thyroiditis is treated with thyroid hormone replacement agents such as levothyroxine (LT₄),<ref name=":6" /> liothyronine (LT₃),<ref name="Ramos-Levi2023" /> or desiccated thyroid extract (T₄+T₃).<ref name=":7" /> In most cases, the treatment needs to be taken for the rest of the person's life.<ref name=":6" />

The standard of care is levothyroxine (LT₄) therapy, which is an oral medication identical in molecular structure to endogenous thyroxine (T₄).<ref name=":6" /> Levothyroxine sodium has a sodium salt added to increase the gastrointestinal absorption of levothyroxine.<ref>Template:Cite book</ref> Levothyroxine has the benefits of a long half-life<ref name="Groenewegen-2021">Template:Cite journal</ref> leading to stable thyroid hormone levels,<ref name="McAninch-2019">Template:Cite journal</ref> ease of monitoring,<ref name="McAninch-2019" /> excellent safety<ref name="McAninch-2019" /><ref>Template:Cite book</ref> and efficacy record,<ref name="Welsh-2016" /> and usefulness in pregnancy as it can cross the fetal blood-brain barrier.<ref name="Klubo-Gwiezdzinska-2022" />

Levothyroxine dosing to normalise TSH is based on the amount of residual endogenous thyroid function and the patient’s weight, particularly lean body mass.<ref name="Klubo-Gwiezdzinska-2022" /> The dose can be adjusted based upon each patient, for example, the dose may be lowered for elderly patients or patients with certain cardiac conditions, but is increased in pregnant patients.<ref name="Mincer2022" /> It is administered on a consistent schedule.<ref name=":6" /> Levothyroxine may be dosed daily or weekly, however weekly dosing may be associated with higher TSH levels, elevated thyroid hormone levels, and transient "echocardiographic changes in some patients following 2-4 h of thyroxine intake".<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Some patients elect combination therapy with both levothyroxine and liothyronine (which is identical in molecular structure to tri-iodothyronine) however studies of combination therapy are limited,<ref name="Ramos-Levi2023" /> and five meta-analyses/reviews "suggested no clear advantage of the combination therapy."<ref name="Klubo-Gwiezdzinska-2022" /> However, subgroup analysis found that patients who remain the most symptomatic while taking levothyroxine may benefit from therapy containing liothyronine.<ref name="Klubo-Gwiezdzinska-2022" />

There is a lack of evidence around the benefits, long-term effects and side effects of desiccated thyroid extract. It is no longer recommended for the treatment of hypothyroidism.<ref name=":7">Template:Cite journal</ref>

Side Effects

Side effects of thyroid replacement therapy are associated with "inadequate or excessive doses."<ref name=":6" /> Symptoms to watch for include, but are not limited to, anxiety, tremor, weight loss, heat sensitivity, diarrhea, and shortness of breath. More worrisome symptoms include atrial fibrillation and bone density loss.<ref name=":6" /> Long term over-treatment is associated with increased mortality and dementia.<ref name="Hegedüs-2022" />

Monitoring

Thyroid Stimulating Hormone (TSH) is the laboratory value of choice for monitoring response to treatment with levothyroxine.<ref name=":4a">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> When treatment is first initiated, TSH levels may be monitored as often as a frequency of every 6–8 weeks.<ref name=":4a" /> Each time the dose is adjusted, TSH levels may be measured at that frequency until the correct dose is determined.<ref name=":4a" /> Once titrated to a proper dose, TSH levels will be monitored yearly.<ref name=":4a" /> The target level for TSH is the subject of debate, with factors like age, sex, individual needs and special circumstances such as pregnancy being considered.<ref name="Taylor-2024" /> Recent studies suggest that adjusting therapy based on thyroid hormone levels (T₄ and/or T₃) may be important.<ref name=":6" />

Monitoring liothyronine treatment or combination treatment can be challenging.<ref name="Taylor-2024" /><ref name="McAninch-2019" /><ref name="Elsevier-2006">Template:Citation</ref> Liothyronine can suppress TSH to a greater extent than levothyroxine.<ref>Template:Cite journal</ref> Short-acting Liothyronine's short half-life can result in large fluctuations of free T₃<ref name="Elsevier-2006" /> over the course of 24 hours.<ref>Template:Cite journal</ref>

Patients may have to adjust their dosage several times over the course of the disease. Endogenous thyroid hormone levels may fluctuate, particularly early in the disease.<ref>Template:Cite journal</ref> Patients may sometimes develop hyperthyroidism, even after long-term treatment.<ref name="Ramos-Levi2023" /> This can be due to a number of factors including acute attacks of destructive thyrotoxicosis (autoimmune attacks on the thyroid resulting in rises in thyroid hormone levels as thyroid hormones leak out of the damaged tissues).<ref name="Dyrka-2024" /><ref name="Ramos-Levi2023" /> This is usually followed by hypothyroidism.<ref name="Ramos-Levi2023" />

Reverse T₃

Measuring reverse tri-iodothyronine (rT₃) is often mentioned in the lay (non-medical) press as a possible marker to inform T₄ or T₃ therapy, "however, there is currently no evidence to support this application" as of 2023.<ref name="Van Uytfanghe-2023" /> Although cited in the lay press as a possible competitor to T₃, it is unlikely that rT₃ causes hypothyroid symptoms by out-competing T₃ for thyroid hormone receptors, as it has a binding affinity 200 times weaker.<ref name="Halsall-2021" /> It is also unlikely that rT₃ causes poor T₄ to T₃ conversion; despite being demonstrated in vivo to have the potential to inhibit DIO-mediated T₄ to T₃ conversion, this is considered improbable at normal body hormone concentrations.<ref name="Halsall-2021">Template:Cite journal</ref>

Persistent Symptoms

Multiple studies have demonstrated persistent symptoms in Hashimoto's patients with normal thyroid hormone levels (euthyroid)<ref name=":6" /><ref name="Taylor-2024">Template:Cite journal</ref><ref name="Klubo-Gwiezdzinska-2022" /><ref name="Groenewegen-2021" /> and an estimated 10%-15% of patients treated with levothyroxine monotherapy are dissatisfied due to persistent symptoms of hypothyroidism.<ref name="Jonklaas-2019">Template:Cite journal</ref><ref name="Hegedüs-2022">Template:Cite journal</ref> Several different hypothesised causes are discussed in the medical literature:<ref name=":2" /><ref name="Groenewegen-2021" /><ref name="Klubo-Gwiezdzinska-2022" />

Low tissue tri-iodothyronine (T₃) hypothesis

Peripheral tissue T₄ to T₃ conversion may be inadequate: Some patients on LT₄ monotherapy may have blood T₃ levels low or below the normal range,<ref name=":6" /><ref name="Taylor-2024" /> and/or may have local T₃ deficiency in some tissues.<ref name="Wiersinga-2014">Template:Cite journal</ref> Although both molecules can have biological effects, thyroxine (T₄) is considered the "storage form" of thyroid hormone with much less effect, while tri-iodothyronine (T₃) is considered the active form used by body tissues.<ref>Template:Cite journal</ref><ref name="Abdalla-2014">Template:Cite journal</ref> Thus the body must convert thyroxine into tri-iodothyronine.<ref name="Abdalla-2014" /> Tri-iodothyronine is produced primarily by conversion in the liver, kidney, skeletal muscle and pituitary gland.<ref>Template:Cite journal</ref>

Adequate conversion requires sufficient levels of the micronutrients zinc,<ref>Template:Cite journal</ref> selenium,<ref name="Winther-2020" /> iron,<ref>Template:Cite journal</ref> and possibly vitamin A.<ref>Template:Cite journal</ref> Conversion rates may decline with age.<ref>Template:Cite journal</ref> Since deiodinase type 2 is necessary for T₄ to T₃ conversion in some peripheral tissues, "patients with DIO2 gene polymorphisms may have variable peripheral T₃ availability", leading to localised hypothyroidism in some tissues.<ref name="Groenewegen-2021" /><ref name="Klubo-Gwiezdzinska-2022" /><ref name="Winther-2020" /> The Thr92Ala DIO2 polymorphism is present in 12–36% of the population.<ref name="Groenewegen-2021" />

For the latter patients, levothyroxine monotherapy may not be sufficient<ref name="Groenewegen-2021" /> and patients may have improvement on combination therapy of T₄ and T₃.<ref name=":6" /><ref name="Winther-2020" /><ref>Template:Cite journal</ref> As standard immunoassay tests can overestimate blood T₄ and T₃ levels, Ultrafiltration LC-MSMS T₄ and T₃ tests may help to identify patients who would benefit from additional T₃.<ref name="Welsh-2016" />

Inadequate markers hypothesis

There is ongoing debate about how to define euthyroidism and whether TSH is its best indicator.<ref name="Jonklaas-2019" /> TSH may be useful to detect poor thyroid output and may reflect the state of thyroid hormones in the hypothalamic-pituitary-thyroid axis, but not the presence of hormones in other body tissues.<ref name="Hegedüs-2022" /><ref name="Taylor-2024" /><ref name="Wiersinga-2014" /> As a result, LT₄ monotherapy may not result in a "truly biochemically euthyroid state."<ref name="Groenewegen-2021" /> Patients may express a preference for "low normal or below normal TSH values"<ref name="Wiersinga-2014" /> and/or T₄ and T₃ monitoring. The monitoring of other biomarkers that reflect the action of thyroid hormone on tissues has also been proposed.<ref name="Klubo-Gwiezdzinska-2022" /><ref>Template:Cite journal</ref><ref name="Hegedüs-2022" />

As immunoassay Free T₃ and Free T4 tests can overestimate levels, particularly at low thyroid hormone levels, hypothyroidism may be undertreated.<ref name="Welsh-2016" /> LC-MSMS tests may provide more reliable measures.<ref name="Welsh-2016" />

Extra-thyroidal effects of autoimmunity hypothesis

It is hypothesised that autoimmunity may play some role in euthyroid symptoms.<ref name="Taylor-2024" /><ref name="Chaker-2022" /><ref name="Groenewegen-2021" /> Hypothesised mechanisms include the proposal that TPO-antibody-producing lymphocytes may travel out of the thyroid to other tissue, creating symptoms and inflammation due to cross-reaction,<ref name="Groenewegen-2021" /><ref name="Guldvog I et al">Template:Cite journal</ref> or "the inflammatory nature of [...] persistently increased circulating cytokine levels."<ref name="Taylor-2024" /> Multiple studies find that antibodies coincide with symptoms even in euthyroid patients,<ref name="Ramos-Levi2023" /><ref name="Groenewegen-2021" /> and higher levels are associated with increased symptoms,<ref name=":6" /> however "the found association does not prove a causality".<ref name="Groenewegen-2021" /> No treatment currently exists for Hashimoto's autoimmunity, although observed wellbeing improvements after surgical thyroid removal are hypothesised to be due to removing the autoimmune stimulus.<ref name="Klubo-Gwiezdzinska-2022" /><ref name="Guldvog I et al" />

Physical and psychosocial co-morbidities hypothesis

It is hypothesised that euthyroid symptoms may not be due to Hashimoto's or hypothyroidism, but some other "physical and psychosocial co-morbidities".<ref name=":2" /><ref name="Hegedüs-2022" />

Improving wellbeing

Some patients may perceive improved wellbeing while in thyrotoxicosis, however overtreatment has risks (known risks for levothyroxine and unknown risks for liothyronine).<ref name="Hegedüs-2022" /> One study demonstrated surgical thyroid removal may substantially improve fatigue and wellbeing,<ref name="Taylor-2024" /><ref name="Ramos-Levi2023" /> see Surgery considerations, below.

Reducing antibodies

It is not established that reducing antithyroid antibodies in Hashimoto's has benefits.<ref name="Chaker-2022">Template:Cite journal</ref><ref name="Klubo-Gwiezdzinska-2022" /><ref name=":0">Template:Cite journal</ref> A systematic review and meta-analysis of selenium trials found that while selenium reduces TPO antibodies, there was a lack of evidence of effects on "disease remission, progression, lowered levothyroxine dose or improved quality of life".<ref name="Winther-2020" />

Selenium,<ref>Template:Cite journal</ref><ref name="Winther-2020" /> vitamin D,<ref>Template:Cite journal</ref> and metformin<ref name="Jia-2020">Template:Cite journal</ref> can reduce thyroid peroxidase antibodies. There is preliminary evidence that levothyroxine,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Template:Needs updatealoe vera juice<ref>Template:Cite journal</ref> and black cumin seed<ref>Template:Cite journal</ref> may reduce thyroid peroxidase antibodies. Metformin can reduce thyroglobulin antibodies.<ref name="Jia-2020" /> It is not established that a gluten-free diet can reduce antibodies when there is no comorbid coeliac disease.<ref name="Szczuko-2022">Template:Cite journal</ref><ref name="Larsen-2022" /> Gluten-free diets have been shown in several studies to reduce antibodies, and in other studies to have no effect, however there were significant confounding issues in these studies, including not ruling out comorbid coeliac disease.<ref name="Szczuko-2022" />

One study found surgical thyroid removal can substantially reduce anti-thyroid antibody levels,<ref name="Taylor-2024" /><ref name="Ramos-Levi2023" /> see Surgery considerations, below.

Surgery considerations

Surgery is not the initial treatment of choice for autoimmune disease, and uncomplicated Hashimoto's thyroiditis is not an indication for thyroidectomy.<ref name="Ramos-Levi2023" /> Patients generally may discuss surgery with their doctor if they are experiencing significant pressure symptoms, or cosmetic concerns, or have nodules present on ultrasound.<ref name="Ramos-Levi2023" /> One well-conducted study of patients with troublesome general symptoms and with anti-thyroperoxidase (anti-TPO) levels greater than 1000 IU/ml (normal <100 IU/ml) showed that total thyroidectomy caused the symptoms to resolve and median anti-thyroid peroxidase levels to reduce from 2232 to 152 IU/mL,<ref name="Ramos-Levi2023" /><ref name="Garber-2019">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> but post-operative complications were higher than expected:<ref name="Taylor-2024" /> infection (4.1%), permanent hypoparathyroidism (4.1%) and recurrent laryngeal nerve injury (5.5%).<ref name=":2">Template:Cite journal</ref>

Other

Zinc may increase free T₃ levels.<ref name="Larsen-2022" /> A small pilot study found Ashwagandha Root may increase T₃ and T4 levels, however, there's a lack of strong evidence of this benefit and Ashwagandha has a potential to cause adrenal insufficiency.<ref name="Larsen-2022" />

As of 2022, there has been only one study of low-dose naltrexone in Hashimoto's, which did not demonstrate efficacy, therefore nothing supports its use; Removing dairy products in those without lactose intolerance has not been found to be supported.<ref name="Larsen-2022">Template:Cite journal</ref> While soy isoflavones have the potential to theoretically affect T₃ and T₄ production, studies in those with sufficient iodine find no effect.<ref name="Larsen-2022" />

Prognosis

Overt, symptomatic thyroid dysfunction is the most common complication, with about 5% of people with subclinical hypothyroidism and chronic autoimmune thyroiditis progressing to thyroid failure every year. Transient periods of thyrotoxicosis (over-activity of the thyroid) sometimes occur, and rarely the illness may progress to full hyperthyroid Graves' disease with active orbitopathy (bulging, inflamed eyes).<ref name=":1">Template:Cite book</ref>

Rare cases of fibrous autoimmune thyroiditis present with severe shortness of breath and difficulty swallowing, resembling aggressive thyroid tumors, but such symptoms always improve with surgery or corticosteroid therapy. Although primary thyroid B-cell lymphoma affects fewer than one in 1000 persons, it is more likely to affect those with long-standing autoimmune thyroiditis,<ref name=":1" /> as there is a 67- to 80-fold increased risk of developing primary thyroid lymphoma in patients with Hashimoto's thyroiditis.<ref>Template:Cite journal</ref>

Myopathy as a result of muscle fibre changes due to thyroid hormone deficiency may take months or years of thyroid hormone treatment to resolve.<ref name="Fariduddin-2024" /><ref>Template:Cite journal</ref>

Anti-thyroid antibodies

Thyroid peroxidase antibodies typically (but not always) decline in patients treated with levothyroxine,<ref name=":0" /> with decreases varying between 10% and 90% after a follow-up of 6 to 24 months.<ref name="Schmidt-2008">Template:Cite journal</ref> One study of patients treated with levothyroxine observed that 35 out of 38 patients (92%) had declines in thyroid peroxidase antibody levels over five years, lowering by 70% on average. 6 of the 38 patients (16%) had thyroid peroxidase antibody levels return to normal.<ref name="Schmidt-2008" />

Children

Many children diagnosed with Hashimoto's disease will experience the same progressive course of the disease that adults do.<ref name="De Luca-2013">Template:Cite journal</ref> However, of children who develop anti-thyroid antibodies and hypothyroidism, up to 50% are later observed to have normal antibodies and thyroid hormone levels.<ref name="Ramos-Levi2023" /> One case of true remission has been observed in a 12-year-old girl. Her thyroid was observed via ultrasound to progress from early inflammation to severe end-stage Hashimoto's thyroiditis with hypothyroidism, and then return to "almost normal with only minimal features of inflammation" and euthyroidism.<ref>Template:Cite journal</ref>

Epidemiology

Hashimoto's Disease is estimated to affect 2% of the world's population.<ref name="Ramos-Levi2023" /><ref name="Chistiakov-2005">Template:Cite journal</ref> About 1.0 to 1.5 in 1000 people have this disease at any time.<ref name="Maitra 2014 The Endocrine System" />

Sex

Anyone may develop this disease, but it occurs between 8<ref name=":6" /> and 15 times more often in women than in men. Some research suggests a connection to the role of the placenta as an explanation for the sex difference.<ref>Template:Cite journal</ref> Other research suggests the difference in prevalence amongst genders is due to the effects of sex hormones.<ref name="Weetman2021"/>

High iodine consumption

Autoimmune thyroiditis has a higher prevalence in societies that have a higher intake of iodine in their diet, such as the United States and Japan, and among people who are genetically susceptible.<ref name="Monaco">Template:Cite book</ref> It is the most common cause of hypothyroidism in areas of sufficient iodine.<ref name="Mincer2022" /> Also, the rate of lymphocytic infiltration increased in areas where the iodine intake was once low, but increased due to iodine supplementation.<ref name="Dayan96"/><ref name="Khattak-2016">Template:Cite journal</ref>

Iodine deficiency disorder is combated using an increase in iodine in a person's diet. When a dramatic change occurs in a person's diet, they become more at-risk of developing hypothyroidism and other thyroid disorders. Treating iodine deficiency disorder with high salt intakes should be done carefully and cautiously as risk for Hashimoto's may increase.<ref name="Khattak-2016" />

Geographic influence of dietary trends

Geography plays a large role in which regions have access to diets with low or high iodine. Iodine levels in both water and salt should be heavily monitored in order to protect at-risk populations from developing hypothyroidism.<ref>Template:Cite journal</ref> Geographic trends of hypothyroidism vary across the world as different places have different ways of defining disease and reporting cases. Populations that are spread out or defined poorly may skew data in unexpected ways.<ref name="Chistiakov-2005" />

North America

Hashimoto's thyroiditis may affect up to 5% of the United States' population.<ref name=":3a">Template:Cite book</ref> Hashimoto's thyroiditis disorder is thought to be the most common cause of primary hypothyroidism in North America.<ref name="Maitra 2014 The Endocrine System" />

Age

Hashimoto's thyroiditis can occur at any age, including children,<ref name="Monaco" /> but more commonly appears in middle age, particularly for men.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Incidence peaks in the fifth decade of life, but patients are usually diagnosed between age 30–50.<ref name="Niddk2021" /><ref name=":3a" /> The highest prevalence from one study was found in the elderly members of the community.<ref name="Vanderpump-2011">Template:Cite journal</ref> It has been shown that the prevalence of positive tests for thyroid antibodies increases with age, "with a frequency as high as 33 percent in women 70 years old or older."<ref name="Dayan96" />

Race

The prevalence of Hashimoto's varies geographically. The highest rate is in Africa, and the lowest in Asia.<ref name="Hu">Template:Cite journal</ref> In the US, the African-American population experiences it less commonly but has greater associated mortality.<ref>Template:Cite news</ref>

Autoimmune diseases

Those that already have an autoimmune disease are at greater risk of developing Hashimoto's as the diseases generally coexist with each other.<ref name="Chistiakov-2005" /> See Causes > Comorbidities, above.

Secular trends

The secular trends of hypothyroidism reveal how the disease has changed over the course of time given changes in technology and treatment options. Even though ultrasound technology and treatment options have improved, the incidence of hypothyroidism has increased according to data focused on the US and Europe. Between 1993 and 2001, per 1000 women, the disease was found varying between 3.9 and 4.89. Between 1994 and 2001, per 1000 men, the disease increased from 0.65 to 1.01.<ref name="Vanderpump-2011" />

History

Also known as Hashimoto's disease, Hashimoto's thyroiditis is named after Japanese physician Hakaru Hashimoto (1881−1934) of the medical school at Kyushu University,<ref>Template:WhoNamedIt</ref> who first described the symptoms of persons with struma lymphomatosa, an intense infiltration of lymphocytes within the thyroid, in 1912 in the German journal called {{#invoke:Lang|lang}}.<ref name="Hir2013"/><ref>Template:Cite journal</ref> This paper was made up of 30 pages and 5 illustrations all describing the histological changes in the thyroid tissue. Furthermore, all results in his first study were collected from four women. These results explained the pathological characteristics observed in these women especially the infiltration of lymphocyte and plasma cells as well as the formation of lymphoid follicles with germinal centers, fibrosis, degenerated thyroid epithelial cells and leukocytes in the lumen.<ref name="Hir2013"/> He described these traits to be histologically similar to those of Mikulic's disease. As mentioned above, once he discovered these traits in this new disease, he named the disease struma lymphomatosa. This disease emphasized the lymphocyte infiltration and formation of the lymphoid follicles with germinal centers, neither of which had ever been previously reported.<ref name="Hir2013"/>

Despite Dr. Hashimoto's discovery and publication, the disease was not recognized as distinct from Reidel's thyroiditis, which was a common disease at that time in Europe. Although many other articles were reported and published by other researchers, Hashimoto's struma lymphomatosa was only recognized as an early phase of Reidel's thyroiditis in the early 1900s. It was not until 1931 that the disease was recognized as a disease in its own right, when researchers Allen Graham et al. from Cleveland reported its symptoms and presentation in the same detailed manner as Hashimoto.<ref name="Hir2013"/>

In 1956, Drs. Rose and Witebsky were able to demonstrate how immunization of certain rodents with extracts of other rodents' thyroid resembled the disease Hakaru and other researchers were trying to describe.<ref name="Hir2013"/> These doctors were also able to describe anti-thyroglobulin antibodies in blood serum samples from these same animals.<ref name="Hir2013" />

Later on in the same year, researchers from the Middlesex Hospital in London were able to perform human experiments on patients who presented with similar symptoms. They purified anti-thyroglobulin antibody from their serum and were able to conclude that these sick patients had an immunological reaction to human thyroglobulin.<ref name="Hir2013" /> From this data, it was proposed that Hashimoto's struma could be an autoimmune disease of the thyroid gland: "Following these discoveries, the concept of organ-specific autoimmune disease was established and HT recognized as one such disease."<ref name="Hir2013" />

Following this recognition, the same researchers from Middlesex Hospital published an article in 1962 in The Lancet that included a portrait of Hakaru Hashimoto.<ref name="Hir2013"/> The disease became more well known from that moment, and Hashimoto's disease started to appear more frequently in textbooks.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Pregnancy

Conception

It is recommended that hypothyroidism be treated with levothyoxine before conception, to prevent adverse effects on the course of the pregnancy and on the development of the child.<ref name="Klubo-Gwiezdzinska-2022" /> In IVF, embryo transfer is improved when hypothyroidism is treated.<ref name="Gaberšček-2011" />

Pregnancy

The Endocrine Society recommends screening in pregnant women who are considered high-risk for thyroid autoimmune disease.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Universal screening for thyroid diseases during pregnancy is controversial, however, one study "supports the potential benefit of universal screening".<ref name="Lepoutre-2012">Template:Cite journal</ref> Pregnant women may have antithyroid antibodies (5%–14% of pregnancies<ref name="Klubo-Gwiezdzinska-2022" />), poor thyroid function resulting in hypothyroidism, or both. Each is associated with risks:<ref name="Klubo-Gwiezdzinska-2022" />

Anti-thyroid antibodies in pregnancy

The presence of Thyroid peroxidase antibodies at the outset of pregnancy are associated with a greater risk to the mother of hypothyroidism and thyroid impairment in the first year after delivery.<ref>Template:Cite journal</ref>

The presence of antibodies is also associated with "a 2 to 4-fold increase in the risk of recurrent miscarriages, and 2 to 3-fold increased risk of preterm birth", however the reason why is unclear. Thyroid peroxidase antibodies are speculated to indicate other autoimmune processes against the placental-fetal unit.<ref name="Klubo-Gwiezdzinska-2022" />

Levothyroxine treatment in euthyroid women with thyroid autoimmunity does not significantly impact the relative risk of miscarriage and preterm delivery, or outcomes with live birth. "Therefore, no strong recommendations regarding the therapy in such scenarios could be made, but consideration on a case-by-case basis might be implemented."<ref name="Klubo-Gwiezdzinska-2022" />

Hypothyroidism in pregnancy.

Women who have low thyroid function that has not been stabilized are at greater risk of complications for both parent and child. Risks to the mother include gestational hypertension including preeclampsia and eclampsia, gestational diabetes, placental abruption, and postpartum hemorrhage.<ref name="Klubo-Gwiezdzinska-2022" /> Risks to the infant include miscarriage, preterm delivery, low birth weight, neonatal respiratory distress, hydrocephalus, hypospadias, fetal death, infant intensive care unit admission, and neurodevelopmental delays (lower child IQ, language delay or global developmental delay).<ref name="Lepoutre-2012" /><ref name="Gaberšček-2011">Template:Cite journal</ref><ref name="Klubo-Gwiezdzinska-2022" />

Successful pregnancy outcomes are improved when hypothyroidism is treated.<ref name="Gaberšček-2011" /> Levothyroxine treatment may be considered at lower TSH levels in pregnancy than in standard treatment.<ref name="Klubo-Gwiezdzinska-2022" /> Liothyronine does not cross the fetal blood-brain barrier, so liothyronine (T₃) only or liothyronine + levothyroxine (T₃ + T₄) therapy is not indicated in pregnancy.<ref name="Klubo-Gwiezdzinska-2022" />

Close cooperation between the endocrinologist and obstetrician benefits the woman and the infant.<ref name="Lepoutre-2012" /><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Immune changes during pregnancy

Hormonal changes and trophoblast expression of key immunomodulatory molecules lead to immunosuppression and fetal tolerance. The main players in regulation of the immune response are Tregs. Both cell-mediated and humoral immune responses are attenuated, resulting in immune tolerance and suppression of autoimmunity. It has been reported that during pregnancy, levels of thyroid peroxidase and thyroglobulin antibodies decrease.<ref name="Weetman-2010" />

Postpartum

Thyroid peroxidase antibodies testing is recommended for women who have ever been pregnant regardless of pregnancy outcome. "[P]revious pregnancy plays a major role in development of autoimmune overt hypothyroidism in premenopausal women, and the number of previous pregnancies should be taken into account when evaluating the risk of hypothyroidism in a young women [sic]."<ref name="Carlé-2014" />

Postpartum thyroiditis can occur in women with Hashimoto's.<ref name="Ramos-Levi2023" /> In healthy women, Postpartum thyroiditis can occur up to 1 year after delivery and should be differentiated from Hashimoto's thyroiditis as it is treated differently.<ref>Template:Cite journal</ref>

After giving birth, Tregs rapidly decrease and immune responses are re-established. It may lead to the occurrence or aggravation of autoimmune thyroid disease.<ref name="Weetman-2010">Template:Cite journal</ref> In up to 50% of females with thyroid peroxidase antibodies in the early pregnancy, thyroid autoimmunity in the postpartum period exacerbates in the form of postpartum thyroiditis.<ref>Template:Cite journal</ref> Higher secretion of IFN-γ and IL-4, and lower plasma cortisol concentration during pregnancy has been reported in females with postpartum thyroiditis than in healthy females. It indicates that weaker immunosuppression during pregnancy could contribute to the postpartum thyroid dysfunction.<ref>Template:Cite journal</ref>

Fetal microchimerism

Several years after the delivery, the chimeric male cells can be detected in the maternal peripheral blood, thyroid, lung, skin, or lymph nodes. The fetal immune cells in the maternal thyroid gland may become activated and act as a trigger that may initiate or exaggerate the autoimmune thyroid disease. In Hashimoto's disease patients, fetal microchimeric cells were detected in thyroid in significantly higher numbers than in healthy females.<ref>Template:Cite journal</ref>

Other animals

Hashimoto's disease is known to occur in chickens, rats, mice, dogs, and marmosets, but Graves' disease does not.<ref name="McLachlan">Template:Cite journal</ref>

See also

• Hashimoto's encephalopathy
• Myxedematous psychosis
• Hashitoxicosis
• Hoffmann Syndrome