While the role of thyroid hormones(TH) in the transcriptional regulation of a wide variety of genes is well documented, its role in the regulation of the GHR and GH-binding protein (GHBP) gene is not known. To study the effect of HYP on the transcription of the GHR & GHBP gene, we induced HYP by thyroidectomy in male(HYP/♂) and female(HYP/♀) rats (n=6 each group) and four wks later compared them to age and sex matched euthyroid control rats. Compared to the control group, the HYP rats had decreased T4 (mean ± SD, 7.0 ± 1.4 vs 2.3 ± 0.8 μg/dl, p <0.01) and increased TSH (63 ± 3 vs 3 ± 0.5 ng/ml, p<0.01). After extraction of total RNA, steady state levels of mRNA for GHR & GHBP in liver were measured by northern analysis and ribonuclease protection assay. Whereas levels of hepatic GHR & GHBP mRNA were decreased (p <0.01) in HYP/♂ rats, levels of hepatic GHR & GHBP mRNA were increased (p <0.01) in HYP/♀ rats when each group was compared to its appropriate controls. These differences could not be attributed to differences in circulating levels of GH (9.8 ± 2.7 vs 13.4 ± 13.1 ng/ml, HYP/♀ vs HYP/♂). We conclude that (1) TH affects the transcription of GHR & GHBP genes, and (2) there is distinct sexual dimorhpism in the effect of HYP on GHR/GHBP gene transcription.