We have previously demonstrated that interferon-gamma (IFN-gamma) induced HLA-DR antigen and also inhibited thyrotropin (TSH)-induced triiodothyronine (T3) and thyroglobulin (Tg) secretion from cultured human thyrocytes. In order to further clarify the inhibitory effect of IFN-gamma on TSH-stimulated thyroid hormone secretion, we have examined human thyroid peroxidase (TPO) gene expression. Thyrocytes dispersed from Graves' thyroid tissues were incubated with TSH with or without IFN-gamma. Total RNA was extracted, separated and hybridized with 32P-labelled human TPO cDNA. Thyrocytes expressed four TPO mRNA transcripts (4.0, 3.2, 2.1 and 1.7kb, respectively), all of which were stimulated by TSH. IFN-gamma inhibited TSH-stimulated TPO mRNA in a dose dependent manner (0.01-10U/mL). 1 U/mL IFN-gamma caused maximal suppression of TSH-stimulated TPO mRNA levels to basal levels. IFN-gamma also inhibited 8-bromo-cyclic AMP-stimulated TPO mRNA levels. In contrast, the gamma-actin mRNA hybridization signal was not altered in control or treated cells. These results demonstrate that IFN-gamma directly inhibits TSH-stimulated TPO gene expression and provide further support for a role of IFN-gamma as a local modulator of thyroid hormone synthesis.