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

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

Hashimoto's thyroiditis is linked with other conditions, including type 1 diabetes, Addison's disease, rheumatoid arthritis, pernicious anemia, and premature menopause.

Treatment may be needed if symptoms of low thyroid production (hypothyroidism) occur 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 needed.