Hashimoto's thyroiditis very often results in hypothyroidism with bouts of hyperthyroidism. Symptoms of Hashimoto's thyroiditis include Myxedematous psychosis, weight gain, depression, mania, sensitivity to heat and cold, paresthesia, fatigue, panic attacks, bradycardia, tachycardia, high cholesterol, reactive hypoglycemia, constipation, migraines, muscle weakness, cramps, memory loss, infertility and hair loss.

The thyroid gland may become firm, large, and lobulated in Hashimoto's thyroiditis, but changes in the thyroid can also be nonpalpable.^[2] Enlargement of the thyroid is due to lymphocytic infiltration and fibrosis rather than tissue hypertrophy. Physiologically, antibodies against thyroid peroxidase (TPO) and/or thyroglobulin cause gradual destruction of follicles in the thyroid gland. Accordingly, the disease can be detected clinically by looking for these antibodies in the blood. It is also characterized by invasion of the thyroid tissue by leukocytes, mainly T-lymphocytes. It is associated with non-Hodgkin lymphoma.

Diagnosis is made by detecting elevated levels of anti-thyroid peroxidase antibodies in the serum.

Given the relatively non-specific symptoms of initial hypothyroidism, Hashimoto's thyroiditis is often misdiagnosed as depression, cyclothymia, PMS, chronic fatigue syndrome, fibromyalgia and, less frequently, as ED or an anxiety disorder. On gross examination, there is often presentation of a hard goitre that is not painful to the touch;^[3] other symptoms seen with hypothyroidism, such as periorbital myxedema, depends on the current state of progression of the response, especially given the usually gradual development of clinically relevant hypothyroidism. Testing for thyroid-stimulating hormone (TSH), Free T3, Free T4, and the anti-thyroglobulin antibodies (anti-Tg), anti-thyroid peroxidase antibodies (anti-TPO) and anti-microsomal antibodies can help obtain an accurate diagnosis.^[4] Earlier assessment of the patient may present with elevated levels of thyroglobulin owing to the transient thyrotoxicosis as inflammation within the thyroid causes damage to the integrity of thyroid follicle storage of thyroglobulin; TSH is concomitantly decreased.^[5]

This exposure of the body to substantial amounts of previously isolated thyroid enzymes is thought to contribute to the exacerbation of tolerance breakdown giving rise to the more pronounced symptoms seen later in the disease as thyroperoxidase and thyroglobulin are further exposed to without. Lymphocytic infiltration of the thyrocyte-associated tissues often leads to the histologically significant finding of germinal center development within the thyroid gland.

Hashimoto's when presenting as mania is known as Prasad's syndrome after Ashok Prasad, the psychiatrist who first described it.^[6]

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. In addition Hashimoto's thyroiditis may be associated with CTLA-4(Cytotoxic T-lymphocyte Associated-4) gene polymorphisms that result in reduced functioning of the gene's products, which are associated with negative regulation of T-lymphocyte activity.^[7] Downregulatory gene polymorphisms affecting CTLA4 are also associated with autoimmune pathology seen in development of Type I diabetes.^[8] The strong genetic component underscoring this theory is born 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.^[9]

Preventable environmental factors, including high iodine intake, selenium deficiency, and pollutants such as tobacco smoke, as well as infectious diseases and certain drugs, have been implicated in the development of autoimmune thyroid disease in genetically predisposed individuals.^[10] The genes implicated vary in different ethnic groups and the incidence is increased in patients with chromosomal disorders, including Turner, Down's, and Klinefelter's syndromes usually associated with autoantibodies against thyroglobulin and thyroperoxidase. Progressive depletion of these cells as the cytotoxic immune response develops leads to higher degrees of primary hypothyroidism, presenting with a poverty of T3/T4 levels, and compensatory elevations of TSH.

There are multiple suggested mechanisms by which the pathology of Hashimoto's Thyroiditis develops. Various autoantibodies may be present against thyroid peroxidase, thyroglobulin and TSH receptors, although a small percentage of patients may have none of these antibodies present. As indicated in various twin studies a percentage of the population may also have these antibodies without developing Hashimoto's thyroiditis. Nevertheless, antibody-dependent cell-mediated cytotoxicity is a substantial factor behind the apoptotic fall-out of Hashimoto's thyroiditis. Activation of cytotoxic T-lymphocytes (CD8+ T-cells) in response to cell-mediated immune response affected by helper T-lymphocytes (CD4+ T-cells) is central to thyrocyte destruction. As is characteristic of type IV hypersensitivities, recruitment of macrophages is another effect of the helper T-lymphocyte activation, with Th1 axis lymphocytes producing inflammatory cytokines within thyroid tissue to further macrophage activation and migration into the thyroid gland for direct effect.

Gross morphological changes within the thyroiditis 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.^[3]

Histologically, the hypersensitivity is seen as diffuse parenchymal infiltration by lymphocytes, particularly plasma B-cells, which can often be seen as secondary lymphoid follicles (germinal centers, not to be confused with the normally present colloid-filled follicles that constitute the thyroid). Atrophy of the colloid bodies is lined by Hürthle cells, cells with intensely eosinophilic, granular cytoplasm, a metaplasia from the normal cuboidal cells that constitute the lining of the thyroid follicles. Severe thyroid atrophy presents often with denser fibrotic bands of collagen that remains within the confines of the thyroid capsule.^[11]