Chronic thyroiditis or Hashimoto's disease is a common thyroid gland disorder that can occur at any age, but it is most often seen in middle aged women. It is caused by a reaction of the immune system against the thyroid gland.

The onset of the disease is slow, and it may take months or even years for the condition to be detected. Chronic thyroiditis is most common in women and individuals with a family history of thyroid disease. It is estimated to affect between 0.1% and 5% of all adults in Western countries.

Hashimoto's disease may rarely be associated with other endocrine disorders caused by the immune system. When Hashimoto's disease occurs with adrenal insufficiency and type 1 diabetes mellitus, the condition is called type 2 polyglandular autoimmune syndrome (PGA II).

Less commonly, Hashimoto's disease occurs with hypoparathyroidism, adrenal insufficiency, and fungal infections of the mouth and nails in a condition called type 1 polyglandular autoimmune syndrome (PGA I).