Effect of propylthiouracil on nycthemeral and sex related variation of plasma TSH in rats.

Studies of the effect on feedback mechanisms in nycthemeral fluctuation of plasma TSH and of its sex-related variation were carried out on normally fed (Purina) and propylthiouracil (PTU) treated rats. Plasma TSH concentration of purina fed rat was significantly higher in males than in females (M = 96 +/- 14, F = 49 +/- 3 microunits/ml, P less than 0.01). Plasma T4 concentration also showed a high tendency in males compared to females (M = 7.4 +/- 0.5, F = 5.8 +/- 1.0 micrograms/dl, 0.05 less than P less than 0.1) but the statistical difference was not significant. Plasma TSH demonstrated a typical periodicity characterized by a zenith at 1200 hr and a nadir at 1800 hr as in the previous reports. Plasma TSH concentrations rose apparently after the diet was switched to the one containing 0.15% of PTU. On the other hand, plasma T4 concentrations decreased to the very low levels following PTU administration. Concerning to the nycthemeral fluctuation and its sex related variation following PTU administration, nycthemeral fluctuation in male rats disappeared rapidly, whereas it was preserved in female rats. These facts showed that it kept more persistent fluctuation in female rats than in male rats under the high plasma TSH levels. Furthermore, female rats showed higher concentration of plasma TSH after PTU treatment compared to male rats. These results suggest that the nycthemeral fluctuation of plasma TSH was abolished or masked by the feedback mechanism of hypothalamo-pituitary-thyroid axis and there was a sex-related difference.