Thyroid hormone increases transcription of growth hormone mRNA in rainbow trout pituitary.

Treatment of young rainbow trout (Oncorhynchus mykiss) fish for one week with thyroid hormone (T3) (3,3',5-triiodo-L-thyronine) increased the steady state levels of mRNA for growth hormone in the pituitary 3-4 times as compared to control groups. The steady state levels of growth hormone in the pituitary and in the plasma, were not significantly altered. This implicates the existence in fish of a molecular mechanism of transcriptional stimulation of the GH gene by thyroid hormone, similar to that observed in the rat. These results may explain part of the known anabolic effects of thyroid hormone in fish.