The thyroid, like any other structure, may be the seat of an acute or chronic suppurative or nonsuppurative inflammation. Various systemic infiltrative disorders may leave their mark on the thyroid gland as well as elsewhere. Infectious thyroiditis is a rare condition, usually the result of bacterial invasion of the gland. Its signs are the classic ones of inflammation: heat, pain, redness, and swelling, and special ones conditioned by local relationships, such as dysphagia and a desire to keep the head flexed on the chest in order to relax the peritracheal muscles. The treatment is that for any febrile disease, including specific antibiotic drugs if the invading organism has been identified and its sensitivity to the drug established. Otherwise, a broad-spectrum antibiotic may be used. Surgical drainage may be necessary and a search for a pyriform sinus fistula should be made, particularly in children with thyroiditis involving the left lobe. Subacute (granulomatous) thyroiditis is a more common and protracted disease that usually involves the thyroid symmetrically. The gland is swollen and tender, and the systemic reaction may be severe, with fever and an elevated erythrocyte sedimentation rate. During the acute phase of the disorder, tests of thyroid function disclose a diminished thyroidal RAIU and increased serum concentrations of T4, T3, and Tg. The cause of this disease has been established in only a few instances in which a viral infection has been the initiating factor. There may be repeated recurrences of diminishing severity. Usually, but not always, the function of the thyroid is normal after the disease has subsided. Subacute thyroiditis may be treated with rest, non-steroidal anti-inflammatory drugs or aspirin, and thyroid hormone. If the disease is severe and protracted, it is usually necessary to resort to administration of glucocorticoids, but recurrence may follow their withdrawal. Riedel's thyroiditis is a chronic sclerosing replacement of the gland that is exceedingly rare. The process involves the immediately adjacent structures, making any surgical attack very difficult. The cause is unknown, and no treatment is available beyond resecting the isthmus of the thyroid gland to relieve the symptoms of tracheal or esophageal compression. Sarcoid may involve the thyroid, and amyloid may be deposited in the gland in quantities sufficient to cause goiter. In all of these diseases it may be necessary to give the patient levothyroxine replacement therapy if the function of the gland has been impaired.