The thyroid gland seen here has an abundant lymphocytic infiltrate with lymphoid follicles most characteristic for Hashimoto thyroiditis. The small thyroid follicles are lined by Hürthle cells with abundant pink cytoplasm. This is an autoimmune condition that accounts for most cases of adult-onset hypothyroidism from disease. There is a female predominance of more than 10:1. There is a genetic predisposition, with disease clusters in families. The initial painless diffuse enlargement of the thyroid may go unnoticed, and eventually over years there is thyroid atrophy, so the onset of hypothyroidism is gradual. Hashimoto thyroiditis results from abnormal T cell activation that stimulates B cells to secrete autoantibodies. Laboratory test findings can include serum autoantibodies to thyroglobulin and thyroid peroxidase.