Regulation of growth hormone expression in fetal rat pituitary gland by thyroid or glucocorticoid hormone

The regulation of growth hormone (GH) cell development by thyroid and glucocorticoid hormones in the fetal rat pituitary gland was examined. Dexamethasone (Dex) treatment of dams induced GH and GH mRNA accumulation in the fetal pituitary gland on day 17 or 18 of gestation when substantial GH expression has not yet occurred in the control fetus. The additional thyroxine injections apparently enhanced the effect of Dex, whereas it exhibited no effect when given alone. The reduction of fetal thyroid hormone level by methimazole suppressed either the Dex induction of GH expression on day 18 or the spontaneous onset of GH expression on day 19 of gestation. The results suggest that 1) thyroid hormone exerts its stimulatory action on fetal GH gene expression only in the presence of glucocorticoid, 2) this synergistic action of these two hormones is evident as early as day 17 of gestation, and 3) rapid maturation of GH cells seen on day 19 in the normal fetus is considered to be induced by concomitant increase in both serum thyroid and glucocorticoid hormone levels.