Regulation of TSH secretion in rats chronically exposed to heat (34 degrees C)

Previous studies have shown that in heat exposed rats, a decreased plasma T4 concentration was associated with a normal biologically active TSH concentration. This study was designed to clarify this apparent discrepancy in the regulation of TSH secretion. In experimental rats (34 degrees C for 25 days) and controls (25 degrees C), plasma total T4 was 3.2 vs. 5.7 x 10(-8) mol/l. (P less than 0.01), plasma total T3: 2.4 vs. 5.7 x 10(-10)mol/l. (P less 0.01) and plasma TSH: bioassay 0.34 vs 0.29 mU/ml (ns), radioimmunoassay: 1.04 vs. 0.87 microgram RP1/ml (ns). After TRH, plasma TSH increased identically in the two groups. In heat-exposed rats, the dialysable fraction of T4 and T3 were were increased: 0.032 vs. 0.020% (P less than 0.05) and 0.102 vs. 0.086% (P less than 0.05), respectively; accordingly, free T4 concentration was normal and that of free T3 was low; total plasma proteins were slightly increased. It is concluded that in heat-exposed rats: (1) plasma thyroid hormone binding activity was decreased as shown by the association: decreased plasma total T4--elevated free T4 fraction. The normality of the free T4 concentration accounted for the normal plasma TSH. (2) the combination of normal plasma TSH, normal plasma free T4, low plasma free T3 concentrations would suggest that T4 is predominantly involved in the regulation of TSH secretion.