Thyrotropin-releasing hormone (TRH) stimulates and thyroid hormone (T3) inhibits transcription of the thyrotropin beta-subunit gene (TSH-beta). The first exon contains DNA sequences necessary for both responses and binds both AP-1 and thyroid hormone receptor (T3R). T3 did not inhibit TSH-beta gene expression in a T3R-deficient cell line. Transfection of a T3R expression vector, however, resulted in a 70% inhibition of expression by T3, which was abolished by cotransfection of c-jun and c-fos expression vectors. Mutations surrounding the transcription initiation site and DNA binding studies demonstrate both a functional and structural interaction between c-jun and T3R. Thus, TRH, acting through AP-1, may alter the set point and magnitude of thyroid hormone negative feedback of the TSH-beta gene through an interaction between AP-1 and T3R. Other regulatory pathways acting through AP-1 may alter thyroid hormone action in man.