From indirect and circumstantial evidence it would appear that subacute thyroiditis is probably a viral infection of the thyroid gland. The thyroid may respond in the stereotyped manner to any one of a number of viruses. Despite the available evidence, however, the viral theory has yet to be finally proven. Autoimmunity does not appear to be a primary factor in the initiation of subacute thyroiditis, although transient immunologic responses occur as a secondary phenomenon. Autoimmune thyroiditis, on the other hand, is almost certainly a primary immunologic disease, and likely is due to a specific inherited defect in immunologic control. It would appear that both cell-mediated immunity and humoral immunity (i.e., T and B lymphocytes) are necessary for the full expression of this disorder.