The TSH, T4, T3 and prolactin responses to consecutive infusions of a potent and stabilized thyrotrophin releasing hormone analogue, RX77368, in man.

The endocrine manifestations of a stabilized thyrotrophin releasing hormone (TRH) analogue, RX77368, have been investigated in six male volunteers. Infusions were given on two occasions, with a 5-day interval between infusions. On the second exposure to RX77368, there was a significant reduction in the TSH response, despite normal basal T3 and T4 levels, while the response of prolactin to RX77368 was unaltered. Domperidone administered during the infusion of RX77368 caused a further elevation of prolactin levels, whilst TSH levels were unchanged. This study shows the differential regulation of thyrotrophs and lactotrophs in response to stimulation by a TRH analogue, and shows, for the first time, down-regulation of the TSH response in vivo, in the presence of normal peripheral thyroid hormone levels. The T3 response to infusions of RX77368 was less than to a bolus injection of TRH, despite a greater TSH response to the analogue, suggesting impaired biological activity of TSH released in response to an infusion of the analogue.