Hashimoto's thyroiditis, also called Hashimoto's disease or chronic autoimmune thyroiditis, is a condition that can cause an underactive thyroid gland (hypothyroidism). Hashimoto's thyroiditis develops when the body's natural defense system (immune system) makes antibodies that attack and eventually destroy the thyroid gland.

Hashimoto's thyroiditis is the most common form of autoimmune thyroid disease and occurs most often in women and older adults. The disease does not cause any pain and often goes unnoticed for years.

Hashimoto's thyroiditis is associated with other conditions, including diabetes, Addison's disease, rheumatoid arthritis, pernicious anemia, and premature menopause.

Treatment may be needed if symptoms of low thyroid production (hypothyroidism) develop or if the thyroid gland becomes inflamed and enlarged. Symptoms of an underactive thyroid gland include fatigue, thinning hair, dry skin, and brittle nails. If the disease does not cause these problems, treatment may not be necessary.