Effect of simultaneous administration of GHRH (1-40) and TRH on GH, PRL and TSH secretion in normal man.

The GHRH test represents a new tool in the study of secretion in man. Nine normal fasting males received on separate occasions in random order 1) GHRH 1-40 (1 microgram/Kg bw) iv at time 0; 2) TRH (6 micrograms/min) infusion between -30 and +120 min; 3) GHRH 1-40 (1 microgram/Kg bw) iv at time 0 plus TRH (6 micrograms/min) infusion between -30 and +120 min. Blood samples were drawn for GH, PRL and TSH at -90, -60, -30, 0 min and then every 15 min for 2 h. GHRH significantly increased GH in all subjects. The same GH response was found during GHRH plus TRH test. No effect was found either on PRL and TSH secretion after GHRH administration, or on GH pattern after TRH administration. A significant decrease of TSH, but not of PRL response was observed after GHRH plus TRH administration in comparison to TRH alone. These results underline that the inhibitory effect exerted by TRH on GH secretion during some experimental conditions is not linked to a pituitary interference between GHRH and TRH. The difference in TSH secretion, following GHRH plus TRH in comparison with TRH alone, could be due to a GHRH-induced central inhibitory mechanism, probably GHRH-related.