Thyroid hormones interact with glucocorticoids to affect somatotroph abundance in chicken embryonic pituitary cells in vitro.

Our laboratory has reported that somatotroph differentiation occurs between d 14 and d 16 of chicken embryonic development and that corticosterone (CORT) can induce somatotroph differentiation at an earlier age in vitro and in vivo. The objective of the present study was to test for thyroid hormone-CORT interactions on somatotroph differentiation in vitro. Pituitary cells from d 11 chicken embryos were treated with CORT and thyroid hormones, and GH-producing somatotrophs were detected by reverse hemolytic plaque assays and immunocytochemistry. We found that thyroid hormones can act synergistically with CORT to further augment the abundance of somatotrophs in vitro but have little to no effect on their own. Both T(4) and T(3) could act synergistically with CORT to increase somatotroph abundance, but the effects of T(3) were biphasic, inhibiting CORT actions at higher concentrations. The monodeiodination inhibitor iopanoic acid inhibited the synergistic effect of T(4) on CORT induction of GH cells in vitro but not the synergistic effect of CORT and T(3) or the effect of CORT alone. Furthermore, T(3) treatment overcame the iopanoic acid-induced reduction in the T(4)-CORT effect. Our findings indicate that thyroid hormones act synergistically with CORT to further augment the abundance of somatotrophs in vitro and that conversion of T(4) to T(3) within the pituitary is involved in T(4) modulation of somatotroph abundance. Somatotroph differentiation during normal development may be regulated by complex interactions of hormones produced by the embryonic thyroid and adrenal glands.