The term "subacute thyroiditis" has come to be applied to two rather distinct forms of inflammation of the thyroid gland, both of which run courses lasting several weeks to several months. While it is clear that the term "subacute thyroiditis" connotes a temporal quality only, and could conceivably apply to any inflammatory process of intermediate severity and duration, by current usage it excludes several forms of infective (ie, bacterial fungal) thyroiditis. On the other hand, it specifically includes a painful form of thyroiditis of probable viral origin (de Quervain or pseudogranulomatous thyroiditis), and a painless form of thyroiditis, which may be called "subacute lymphocytic thyroiditis." The latter term does not necessarily imply that there is any relationship to chronic lymphocytic thyroiditis, as will be discussed carefully below.