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.” The mean age of prevalence in women is higher than in men by one year, (58 and 59 years old respectively).

Autoimmune thyroiditis can affect children. It is very rare in children under the age of five, but can occur;it accounts for around 40 percent of cases in adolescents with goiters.

People with hypothyroidism over the age of 40 have an increased chance of developing autoimmune thyroiditis.

Pregnant women who are positive for Hashimoto's thyroiditis may have decreased thyroid function or the gland may fail entirely. If a woman is TPOAb-positive, clinicians can inform her of the risks for themselves and their infants if they go untreated. "Thyroid peroxidase antibodies (TPOAb) are detected in 10% of pregnant women," which presents risks to those pregnancies. Women who have low thyroid function that has not been stabilized are at greater risk of having an infant with: low birth weight, neonatal respiratory distress, hydrocephalus, hypospadias, miscarriage, and preterm delivery. The embryo transplantion rate and successful pregnancy outcomes are improved when Hashimoto's is treated. Recommendations are to only treat pregnant women who are TPOAb-positive throughout the entirety of their pregnancies and to screen all pregnant women for thyroid levels. Close cooperation between the endocrinologist and obstetrician benefits the woman and the infant. The Endocrine Society recommends screening in pregnant women who are considered high-risk for thyroid autoimmune disease.

Thyroid peroxides 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"]."

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. Also, the rate of lymphocytic infiltration increased in areas where the iodine intake was once low, but increased due to iodine supplementation. “The prevalence of positive serum tests in such areas rises to over 40 percent within 0.5 to 5 years.”

The strong genetic component is borne out in studies on monozygotic twins, 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 aetiology.

Hashimoto's thyroiditis is associated with "CTLA-4" ("Cytotoxic T-lymphocyte Antigen-4") gene polymorphisms. CTLA-4 downregulates., i.e. transmits an inhibitory signal to T cells so reduced functioning is associated with increased T-lymphocyte activity. A family history of thyroid disorders is common, with the "HLA-DR5" gene most strongly implicated conferring a relative risk of 3 in the UK.

Having other autoimmune diseases is a risk factor to develop Hashimoto’s thyroiditis, and the opposite is also true. Autoimmune diseases most commonly associated to Hashimoto’s thyroiditis include celiac disease, type 1 diabetes, vitiligo, and alopecia.

Preventable environmental factors, including high iodine intake, selenium deficiency, as well as infectious diseases and certain drugs, have been implicated in the development of autoimmune thyroid disease in genetically predisposed individuals.

The genes implicated vary in different ethnic groups and the incidence is increased in people with chromosomal disorders, including Turner, Down, and Klinefelter syndromes usually associated with autoantibodies against thyroglobulin and thyroperoxidase. Progressive depletion of these cells as the cytotoxic immune response leads to higher degrees of primary hypothyroidism, presenting with a poverty of T3/T4 levels, and compensatory elevations of TSH.

Most types of thyroiditis are three to five times more likely to be found in women than in men. The average age of onset is between thirty and fifty years of age. This disease tends to be geographical and seasonal, and is most common in summer and fall.

A person's sex also seems to have some role in the development of autoimmunity; that is, most autoimmune diseases are "sex-related". Nearly 75% of the more than 23.5 million Americans who suffer from autoimmune disease are women, although it is less-frequently acknowledged that millions of men also suffer from these diseases. According to the American Autoimmune Related Diseases Association (AARDA), autoimmune diseases that develop in men tend to be more severe. A few autoimmune diseases that men are just as or more likely to develop as women include: ankylosing spondylitis, type 1 diabetes mellitus, granulomatosis with polyangiitis, Crohn's disease, Primary sclerosing cholangitis and psoriasis.

The reasons for the sex role in autoimmunity vary. Women appear to generally mount larger inflammatory responses than men when their immune systems are triggered, increasing the risk of autoimmunity. Involvement of sex steroids is indicated by that many autoimmune diseases tend to fluctuate in accordance with hormonal changes, for example: during pregnancy, in the menstrual cycle, or when using oral contraception. A history of pregnancy also appears to leave a persistent increased risk for autoimmune disease. It has been suggested that the slight, direct exchange of cells between mothers and their children during pregnancy may induce autoimmunity. This would tip the gender balance in the direction of the female.

Another theory suggests the female high tendency to get autoimmunity is due to an imbalanced X chromosome inactivation. The X-inactivation skew theory, proposed by Princeton University's Jeff Stewart, has recently been confirmed experimentally in scleroderma and autoimmune thyroiditis. Other complex X-linked genetic susceptibility mechanisms are proposed and under investigation.

Treatments for this disease depend on the type of thyroiditis that is diagnosed. For the most common type, which is known as Hashimoto's thyroiditis, the treatment is to immediately start hormone replacement. This prevents or corrects the hypothyroidism, and it also generally keeps the gland from getting bigger. However, Hashimoto's thyroiditis can initially present with excessive thyroid hormone being released from the thyroid gland (hyperthyroid). In this case the patient may only need bed rest and non-steroidal anti-inflammatory medications; however, some need steroids to reduce inflammation and to control palpitations. Also, doctors may prescribe beta blockers to lower the heart rate and reduce tremors, until the initial hyperthyroid period has resolved.

Outside Europe a goitrous form of autoimmune thyroiditis (Hashimoto's Thyroiditis) is more common than Ord's disease.

Riedel's thyroiditis is classified as rare. Most patients remain euthyroid, but approximately 30% of patients become hypothyroid and very few patients are hyperthyroid. It is most seen in women.

Treatment is as with hypothyroidism, daily thyroxine(T4) and/or triiodothyronine(T3).

Among the complications discussed above, women with anti-Ro/SS-A and anti-La/SS-B antibodies who become pregnant, have an increased rate of neonatal lupus erythematosus with congenital heart block requiring a pacemaker. Type I cryoglobulinemia is a known complication of SS.

Published studies on the survival of SS patients are limited in varied respects, perhaps owing to the relatively small sample sizes, and secondary SS is associated with other autoimmune diseases. However, results from a number of studies indicated, compared to other autoimmune diseases, SS is associated with a notably high incidence of malignant non-Hodgkin lymphoma (NHL). NHL is the cancer derived from white blood cells. About 5% of patients with SS will develop some form of lymphoid malignancy. Patients with severe cases are much more likely to develop lymphomas than patients with mild or moderate cases. The most common lymphomas are salivary extranodal marginal zone B cell lymphomas (MALT lymphomas in the salivary glands) and diffuse large B-cell lymphoma.

Lymphomagenesis in primary SS patients is considered as a multistep process, with the first step being chronic stimulation of autoimmune B cells, especially B cells that produce rheumatoid factor at sites targeted by the disease. This increases the frequency of oncogenic mutation, leading to any dysfunction at checkpoints of autoimmune B-cell activation to transform into malignancy. A study's finding has concluded the continuous stimulation of autoimmune B cells, leading to subtle germinal abnormalities in genes having specific consequences in B cells, which underlies the susceptibility to lymphoma.

Apart from this notably higher incidence of malignant NHL, SS patients show only modest or clinically insignificant deterioration in specific organ-related function, which explains the only slight increases in mortality rates of SS patients in comparison with the remainder of the population.

Hypothyroidism is a state in which the body is not producing enough thyroid hormones, or is not able to respond to / utilize existing thyroid hormones properly. The main categories are:

- Thyroiditis: an inflammation of the thyroid gland

- Hashimoto's thyroiditis / Hashimoto's disease

- Ord's thyroiditis

- Postpartum thyroiditis

- Silent thyroiditis

- Acute thyroiditis

- Riedel's thyroiditis (the majority of cases do not affect thyroid function, but approximately 30% of cases lead to hypothyroidism)

- Iatrogenic hypothyroidism

- Postoperative hypothyroidism

- Medication- or radiation-induced hypothyroidism

- Thyroid hormone resistance

- Euthyroid sick syndrome

- Congenital hypothyroidism: a deficiency of thyroid hormone from birth, which untreated can lead to cretinism

It is named for Fritz de Quervain. It should not be confused with De Quervain syndrome.

Some cases may be viral in origin, perhaps preceded by an upper respiratory tract infection. Viral causes include Coxsackie virus, mumps and adenoviruses. Some cases develop postpartum .

Hyperthyroidism is a state in which the body is producing too much thyroid hormone. The main hyperthyroid conditions are:

- Graves' disease

- Toxic thyroid nodule

- Thyroid storm

- Toxic nodular struma (Plummer's disease)

- Hashitoxicosis: "transient" hyperthyroidism that can occur in Hashimoto's thyroiditis

According to the hygiene hypothesis, high levels of cleanliness expose children to fewer antigens than in the past, causing their immune systems to become overactive and more likely to misidentify own tissues as foreign, resulting in autoimmune conditions such as asthma.

During pregnancy, immunologic suppression occurs which induces tolerance to the presence of the fetus. Without this suppression, the fetus would be rejected causing miscarriage. As a result, following delivery, the immune system rebounds causing levels of thyroids antibodies to rise in susceptible women.

Specifically, the immunohistological features of susceptible women are indicated by:

- antibodies to thyroglobulin (TgAb)

- antibodies to thyroid peroxidase (TPOAb)

- increase in TPOAb subclasses IgG-IgG

- lymphocyte infiltration and follicle formation within thyroid gland (Hashimoto's thyroiditis)

- T-cell changes (increased CD4:CD8 ratio)

- TSH-receptor antibodies (TSH-R Abs)

Subacute lymphocytic thyroiditis is a form of thyroiditis that is also known as silent thyroiditis or painless thyroiditis. Subacute lymphocytic thyroiditis may occur at any age and is more common in females.

A variant of subacute lymphocytic thyroiditis occurs postpartum, postpartum thyroiditis. Both of these entities can be considered subtypes of Hashimoto's thyroiditis and have an autoimmune basis. Anti-thyroid antibodies are common in all three and the underlying histology is similar. This disorder should not be confused with de Quervain's thyroiditis which is another form of subacute thyroiditis.

Subacute thyroiditis is a form of thyroiditis that can be a cause of both thyrotoxicosis and hypothyroidism. It is uncommon and can affect individuals of both sexes and people of all ages. The most common form, subacute granulomatous, or de Quervain's, thyroiditis manifests as a sudden and painful enlargement of the thyroid gland accompanied with fever, malaise and muscle aches. Indirect evidence has implicated viral infection in the aetiology of subacute thyroiditis. This evidence is limited to preceding upper respiratory tract infection, elevated viral antibody levels, and both seasonal and geographical clustering of cases. There may be a genetic predisposition.

Nishihara and coworkers studied the clinical features of subacute thyroiditis in 852 mostly 40- to 50-year-old women in Japan. They noted seasonal clusters (summer to early autumn) and most subjects presented with neck pain. Fever and symptoms of thyrotoxicosis was present in two thirds of subjects. Upper respiratory tract infections in the month preceding presentation were reported in only 1 in 5 subjects. Recurrent episodes following resolution of the initial episode were rare, occurring in just 1.6% of cases. Laboratory markers for thyroid inflammation and dysfunction typically peaked within one week of onset of illness.

Types include:

- Subacute granulomatous thyroiditis (De Quervain thyroiditis)

- Subacute lymphocytic thyroiditis

- Postpartum thyroiditis

- Palpation thyroiditis

Treatment is based on symptoms. Beta-blockers relieve rapid heart rate and excessive sweating during the hyperthyroid phase.

An interesting inverse relationship exists between infectious diseases and autoimmune diseases. In areas where multiple infectious diseases are endemic, autoimmune diseases are quite rarely seen. The reverse, to some extent, seems to hold true. The hygiene hypothesis attributes these correlations to the immune manipulating strategies of pathogens. While such an observation has been variously termed as spurious and ineffective, according to some studies, parasite infection is associated with reduced activity of autoimmune disease.

The putative mechanism is that the parasite attenuates the host immune response in order to protect itself. This may provide a serendipitous benefit to a host that also suffers from autoimmune disease. The details of parasite immune modulation are not yet known, but may include secretion of anti-inflammatory agents or interference with the host immune signaling.

A paradoxical observation has been the strong association of certain microbial organisms with autoimmune diseases.

For example, "Klebsiella pneumoniae" and coxsackievirus B have been strongly correlated with ankylosing spondylitis and diabetes mellitus type 1, respectively. This has been explained by the tendency of the infecting organism to produce super-antigens that are capable of polyclonal activation of B-lymphocytes, and production of large amounts of antibodies of varying specificities, some of which may be self-reactive (see below).

Certain chemical agents and drugs can also be associated with the genesis of autoimmune conditions, or conditions that simulate autoimmune diseases. The most striking of these is the drug-induced lupus erythematosus. Usually, withdrawal of the offending drug cures the symptoms in a patient.

Cigarette smoking is now established as a major risk factor for both incidence and severity of rheumatoid arthritis. This may relate to abnormal citrullination of proteins, since the effects of smoking correlate with the presence of antibodies to citrullinated peptides.

Risk factors of progressive and severe thyroid-associated orbitopathy are:

- Age greater than 50 years

- Rapid onset of symptoms under 3 months

- Cigarette smoking

- Diabetes

- Severe or uncontrolled hyperthyroidism

- Presence of pretibial myxedema

- High cholesterol levels (hyperlipidemia)

- Peripheral vascular disease

Riedel's thyroiditis is characterized by a replacement of the normal thyroid parenchyma by a dense fibrosis that invades adjacent structures of the neck and extends beyond the thyroid capsule. This makes the thyroid gland stone-hard (woody) and fixed to adjacent structures. The inflammatory process infiltrates muscles and causes symptoms of tracheal compression. Surgical treatment is required to relieve tracheal or esophageal obstruction.

Worldwide reporting of postpartum thyroiditis cases is highly varied. This variation may be due to methodological discrepancies in assessing women for this condition. Factors such as length of follow-up after delivery, diagnostic criteria, frequency of postpartum blood sampling, and thyroid hormone assay methodology likely contribute to this variation. On average, 5-7% of pregnant women from most iodine-replete populations develop this condition.

Women with type I diabetes mellitus have a threefold increase in the prevalence of postpartum thyroiditis than non-diabetic women in the same region.