Regulation of interleukin-6 release by human thyrocytes

Autoimmune thyroiditis is characterized by lymphocytic accumulation within the thyroid which may be the result, in part, of immunomodulatory cytokine secretion by thyrocytes. We have tested human thyroid cell cultures (n = 9) for interleukin-6 (IL-6) release using two bioassays. IL-6 was detected in all culture supernatants under basal conditions and was increased by gamma-interferon, tumour necrosis factor and TSH in a dose-dependent manner. The bioactivity was confirmed as IL-6 by immunoblotting experiments and could not be accounted for by contamination of the thyroid cell cultures with fibroblasts, lymphocytes or monocytes. Circulating IL-6 levels were not raised in patients with Graves' hyperthyroidism. Exogenous recombinant IL-6 reduced cyclic AMP production in response to TSH when added to thyroid cell cultures. Since IL-6 plays a major role in B cell differentiation and T cell activation, release of IL-6 by thyrocytes may increase the intrathyroidal autoimmune response in Graves' disease and Hashimoto's thyroditis. Our results also suggest that IL-6 may modulate thyroid cell function.