Endogenous thyroid hormones modulate pituitary somatotroph differentiation during chicken embryonic development

Growth hormone cell differentiation normally occurs between day 14 and day 16 of chicken embryonic development. We reported previously that corticosterone (CORT) could induce somatotroph differentiation in vitro and in vivo and that thyroid hormones could act in combination with CORT to further augment the abundance of somatotrophs in vitro. The objective of the present study was to test our hypothesis that endogenous thyroid hormones regulate the abundance of somatotrophs during chicken embryonic development. Plasma samples were collected on embryonic day (e) 9-14. We found that plasma COR-T and thyroid hormone levels increased progressively in mid-embiyogenesis to e13 or e14, immediately before normal somatotroph differentiation. Administration of thyroxine (T,) and triiodothyronine (T-3) into the albumen of fertile eggs on ell increased somatotroph proportions prematurely on e13 in the developing chick embryos in vivo. Furthermore, administration of methimazole, the thyroid hormone synthesis inhibitor, on e9 inhibited somatotroph differentiation in vivo, as assessed on e14; this suppression was completely reversed by T, replacement on ell. Since we reported that T-3 alone was ineffective in vitro, we interpret these findings to indicate that the effects of treatments in vivo were due to interactions with endogenous glucocorticoids. These results indicate that treatment with exogenous thyroid hormones can modulate somatotroph abundance and that endogenous thyroid hormone synthesis likely contributes to normal somatotroph differentiation.