Autoimmune thyroid disease is associated with enhanced expression of major histocompatibility complex class I antigens on thyrocytes. To better understand this phenomenon, we have studied the normal expression of class I genes in FRTL-5 rat thyroid cells. A variety of hormones and growth factors that regulate the growth and function of these thyroid cells were found to decrease class I RNA levels: serum, insulin or insulin-like growth factor-I (IGF-I), and hydrocortisone. Antibody preparations from Graves' patients (thyroid-stimulating antibodies), which increase cAMP levels and stimulate the thyroid, also decrease class I RNA levels. This is consistent with the fact that TSH, via its cAMP signal, reduces class I transcripts. The class I response to TSH, serum, insulin, IGF-I, or hydrocortisone is specific, in that the same agents do not similarly affect TSH receptor, thyroglobulin, thyroid peroxidase, malic enzyme, or beta-actin RNA levels. Both gamma- and alpha-interferon increase class I RNA levels in FRTL-5 cells, even in the presence of the serum, IGF-I, or hormones noted above, i.e. they overcome hormonal negative regulation in normal thyrocytes. In contrast, methimazole treatment of rat FRTL-5 thyroid cells, but not rat fibroblasts or rat FRT thyroid cells, which have no TSH receptor and no TSH-regulated function, results in reduced class I RNA levels. The action of methimazole can inhibit interferon action, is transcriptional, is duplicated by iodide, and is additive with the negative regulatory action of hormones and serum factors, including TSH.