Evidence of direct thyroid-stimulating action of thyrotropin-releasing hormone by perifusion of rat thyroid fragments.

The possibility that TRH has a direct thyroid-stimulating action has never been reported. A number of studies have shown a rise in serum concentrations of thyroid hormone after stimulation with TRH, without a rise in TSH secretion. This has led us to test TRH with rat thyroid fragment perifusion. Significant T4 release was observed for TRH concentrations as low as 1.7 X 10(-11) M. A dose-response curve was determined. The response was immediate, reaching a peak after the sixth minute and continuing for 15 min after the stimulation had ceased. This kinetic pattern is different from the one observed with TSH and with theophylline and suggests that a different mechanism may be involved. TRH seems to be capable of directly stimulating both the secretion of the pituitary hormone (TSH) and the corresponding peripheral gland, like LHRH, which also acts directly on the Leydig cells of the testis.