Hashimoto thyroiditis, or chronic autoimmune thyroiditis, is an autoimmune-mediated destruction of the thyroid gland leading to apoptosis of thyroid epithelial cells. Hashimoto thyroiditis is the most common cause of hypothyroidism in iodine-sufficient areas, and it primarily affects women between 30 and 50 years of age, but can occur in men (female:male ratio of 7:1) and children. This summary addresses Hashimoto thyroiditis in adults.

There are 2 phenotypic forms: goitrous autoimmune thyroiditis and atrophic autoimmune thyroiditis. For both forms, lymphocytic infiltration is seen on pathology with concentrations of antibodies to thyroid peroxidase (TPO) and thyroglobulin (Tg) elevated in the serum.

Because symptom onset may be gradual, patients are initially asymptomatic but may present with a goiter in time. As the disease progresses, signs and symptoms of hypothyroidism such as bradycardia, weight gain, constipation, brittle hair, and fatigue become more apparent.