Thyroid-stimulating hormone responses after single administration of thyrotropin-releasing hormone and combined administration of four hypothalamic releasing hormones in beagle dogs

Thyrotropin (TSH) responses were determined in eight healthy male beagle dogs after a single administration of thyrotropin-releasing hormone (TRH) and the combined administration of four hypothalamic releasing hormones, i.e., corticotropin-releasing hormone, growth hormone-releasing hormone, gonadotropin-releasing hormone, and TRH. In both tests, TRH was administered in a dose of 10 mu g/kg. Basal TSH concentrations ranged form 0.07 to 0.27 mu g/l (mean +/- SE, 0.14 +/- 0.02 mu g/l). The administration of TRH, alone or in the combined test, resulted in a prompt and significant increase in TSH with mean (+/-SE) plasma TSH peaks of 1.26 +/- 0.22 mu g/l at 10 min and 0.85 +/- 0.17 mu g/l at 30 min, respectively. The area under the curve (0-120 min) was significantly lower in the combined test than in the single TRH test, whereas the increments were not significantly different. It is concluded that measurements of TSH responses to TRH alone and in combination with other releasing hormones can be used for the assessment of pituitary thyrotropic cell function. In the combined test, the TSH response is slightly lower than that in the single test.