The thyroid gland may be the seat of an acute or chronic suppurative or non-suppurative inflammation. Infectious thyroiditis is a rare condition, usually the result of bacterial invasion of the gland. Its signs are the classical 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 para-tracheal muscles. Important to differentiate from the acute bacterial infection of acute suppurative thyroiditis (AST) is subacute (granulomatous) thyroiditis (SAT), which is far more common than AST and is characterized by a more protracted course, usually involving the thyroid symmetrically. Compared to SAT, the patient with AST appears septic, has greater and more localized pain in the thyroid gland, may have an associated upper respiratory infection, has lymphadenopathy, and may be immuno-compromised. Acute suppurative thyroiditis should be identified early to allow for appropriate management and prevention of complications and death. Treatment of AST is by specific antibiotic drugs, according to culture and sensitivity, if the invading organism has been identified. Otherwise, a broad-spectrum antibiotic may be used, and a search for a pyriform sinus fistula should be made, particularly in children with thyroiditis involving the left lobe. When an abscess is identified, surgical incision and drainage is essential. Occasionally, complete resection of an affected thyroid lobe is required. Another cause of acute thyroiditis is radiation-induced thyroiditis, which is rare with an incidence of about 0.1–0.7%. The thyroid gland is extremely susceptible to radiation and can be compromised in a wide radiation dose range, especially in higher doses.