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

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Abstract
Studies of the effect on feedback mechanisms in nychthemeral fluctuation of plasma TSH [thyrotropin] 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 [M] than in females [F] (M = 96 .+-. 14, F = 49 .+-. 3 .mu.u/ml, P < 0.01). Plasma T4 [thyroxine] concentration also showed a high tendency in M compared to F (M = 7.4 .+-. 0.5, F = 5.8 .+-. 1.0 .mu.g/dl, 0.05 < P < 0.01) but the statistical difference was not significant. Plasma TSH demonstrated a typical periodicity characterized by a zenith at 1200 h and a nadir at 1800 h as in the previous reports. Plasma TSH concentrations rose apparently after the diet was switched to the one containing 0.15% of PTU. Plasma T4 concentrations decreased to the very low levels following PTU administration. Concerning 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. It kept more persistent fluctuation in female rats than in male rats under the high plasma TSH levels. Female rats showed higher concentration of plasma TSH after PTU treatment compared to male rats. 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.