The syndromes of thyroiditis include five disorders. Hashimoto's thyroiditis, the commonest, is an autoimmune disease whose principal manifestations are goiter and hypothyroidism. Subacute granulomatous thyroiditis is probably viral in origin and usually presents with a tender goiter. Subacute lymphocytic thyroiditis is of unknown pathogenesis, but the postpartum form may be autoimmune. Its principal manifestations are goiter and spontaneously reversible hyperthyroidism. Acute suppurative thyroiditis results from bacterial or fungal infection causing abscess. Riedel's struma, a disease of unknown cause, presents with a goiter and thoracic inlet obstruction. Thyroiditis may require differentiation from other diseases. The goiter may resemble that of Graves' disease or thyroid lymphoma. Thyroid nodules may resemble neoplasms. Hyperthyroidism may suggest Graves' disease or other hyperthyroid syndromes with low radioactive iodine uptake. Neck pain can also occur with some thyroid malignancies. Local abscess is usually infectious but may be undifferentiated thyroid carcinoma. Finally, hypothyroidism may be transient or permanent.