Hashimoto's thyroiditis is the most common form of thyroiditis, an autoimmune disease where the body's own antibodies fight the cells of the thyroid. It is named after the Japanese physician, Hakaru Hashimoto , who first described it in 1912. It is four times more common among women than men, and runs in families, with the HLADR5 gene most strongly implicated (conferring a relative risk of 3) in the UK. The genes implicated vary in different ethnic groups.

In many cases, Hashimoto's usually results in hypothyroidism, although in its acute phase, it can cause a transient hyperthyroid state.

Physiologically, antibodies to thyroid peroxidase 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 characterised by invasion of the thyroid tissue by leukocytes, chiefly T-lymphocytes.

Treatment is by daily thyroxine, with the sodium salt of thyroxine liothyronine given when the need to raise levels of circulating thyroxine is urgent.

Symptoms of Hashimoto's thyroiditis include symptoms of hypothyroidism and a goitre.

In European countries an atrophic form of autoimmune thyroditis (Ord's Thyroiditis) is more common than Hashimoto's disease.