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The secretion of thyrotropin (TSH) has been investigated in the chronically catheterized ovine fetus (term 145-150 days). Forty-two random plasma samples from 25 fetuses (86-149 days of gestation) were measured for TSH concentrations by radioimmunoassay. Plasma TSH concentrations were highest in the youngest fetuses [86-110 days, 3.9 +/- (SD) 5.5 microU/ml, n = 13]. Thereafter TSH concentrations declined to 0.4 +/- 0.6 microU/ml (n = 13, p less than 0.05) at 130-150 days of gestation. However, serial sampling at 15-20 min intervals for 180 min from 14 individual fetuses (91-139 days) showed that TSH was secreted in a markedly exaggerated pulsatile manner compared to that observed after birth. The mean amplitude of TSH pulses fell (p less than 0.005) from 5.9 +/- 8.1 microU/ml in the fetuses to 2.1 +/- 1.1 microU/ml in five neonatal lambs (6-22 days) and to 1.5 +/- 0.4 microU/ml in three adult nonpregnant ewes. The mean pulse frequency for the 14 fetuses was 0.7 +/- 0.3 pulses/h and was reduced (p less than 0.001) to 0.3 +/- 0.1 pulses/h in lambs and to 0.3 +/- 0.1 pulses/h in the ewes. In the neonate, hypothermia is a potent stimulus to TSH release. To examine the ontogeny of this response, the temperature of the fetus in utero (106-127 days of gestation) was lowered by circulating water (14-18 degrees C) at either a fast or slow rate through a coil placed either externally around the fetus or internally in the fetal esophagus and stomach.(ABSTRACT TRUNCATED AT 250 WORDS)

Circulating Thyrotropin in the Ovine Fetus: Evidence for Pulsatile Release and the Effect of Hypothermia in Utero