Biphasic inhibitory and stimulatory effects of thyrotropin on the mitotic activity of thyroid explants cultured in vitro.

An influence of thyrotropin (TSH), dibutyryl cyclic AMP (dbcAMP), triiodothyronine (T3) and thyroxine (T4) on the mitotic incidence in the organ-cultured rat thyroid was studied. The study has revealed biphasic inhibitory and stimulatory effects of TSH on the thyroid mitotic activity. The inhibitory effect occurred 14 h after an exposure of the thyroid explants to TSH and was enhanced by T3. A similar inhibitory effect was observed after an exposure of the explants to dbcAMP. 24 h after the exposure of the thyroid explants to TSH a stimulatory effect on the mitotic incidence was observed. The latter effect was not reproduced by dbcAMP.