Thyroiditis. Acute, subacute, and chronic. Inflammatory diseases of the thyroid are collectively the commonest thyroid disorder. Individually, they range from the rare case of acute bacterial thyroiditis to the other end of the spectrum, the even rarer Riedel's thyroiditis. Relatively common thyroid inflammatory diseases include the subacute thyroiditis syndromes. Of particular interest to endocrinologists is that both subacute granulomatous (painful) thyroiditis and subacute lymphocytic (painless) thyroiditis are very similar in terms of clinical course, although most likely have different etiologies. Nevertheless, their similarities suggest the possibility that there may be etiologic heterogeneity for the syndromes. From a clinical standpoint, it is essential to differentiate subacute painless thyroiditis from Graves' disease, because these two disorders also may mimic each other, yet only Graves' disease requires specific therapy. Chronic lymphocytic (Hashimoto's) thyroiditis, the commonest of the thyroiditides, presents with goiter and either hyperthyroidism (uncommon), hypothyroidism (common), or euthyroidism (most common). When L-T4 therapy is used in the treatment of Hashimoto's thyroiditis, the physician must be alert to the possibility of excess thyroid hormone administration. Sensitive TSH measurements help to avoid this therapeutic pitfall. 