Thyroiditis may be categorized as acute (suppurative), subacute (granulomatous or lymphocytic), or chronic (invasive fibrous or lymphocytic). Acute suppurative thyroiditis is typically caused by a bacterial infection and resolves with appropriate antibiotic treatment. The subacute thyroiditides are characterized by spontaneously resolving hyperthyroidism associated with low radioiodine uptake, often followed by transient hypothyroidism. Neck pain is the initial symptom in subacute granulomatous thyroiditis, and the disorder recurs only in a minority of patients. Subacute lymphocytic thyroiditis is typically painless, often occurs in the postpartum period, and is being increasingly recognized in the Great Lakes area of the United States. Invasive fibrous thyroiditis (Riedel's struma) is exceedingly rare, often mimics carcinoma, and is associated with extracervical foci of fibrosclerosis. Chronic lymphocytic (Hashimoto's) thyroiditis, an organ-specific autoimmune disease, occurs in at least 2% of women. Although the disorder often produces hypothyroidism, the type of thyroid dysfunction present in patients with Hashimoto's disease reflects the character of the dominant thyroid autoantibody--that is, destructive, blocking, or stimulatory.