Nyctohemeral and Sex-Related Variations in Plasma Thyrotropin, Thyroxine, and Triiodothyronine

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Abstract
TSH [thyrotropin], T4 [thyroxine] and T3 [triiodothyronine] were measured by radioimmunoassay in plasma samples obtained from 77 young adult male and 114 female rats fed a Purinea high-iodine diet and maintained in an isolated room 2-4/cage, at 24 .+-. 1.degree. C with light from 0600-1800 h. In 1 experiment, 7 male and 7 female rats were decapitated every 3 h for 30 consecutive hours and trunk blood was collected. There was a clear nyctohemeral rhythm of plasma TSH in both sexes characterized by a zenith at 1200 h and a nadir between 1800 and 2100 h. The plasma TSH cycle was approximately 180.degree. out of phase and negatively correlated (P < .05) with that of plasma corticosterone (B) in both sexes. Although glucocorticoids were reported to suppress TSH secretion, there was no causal relationhip between plasma B and TSH in the experiments since the TSH cycles were normal in chronically adrenalectomized rats. Normal TSH cyclicity was not observed in severly I-deficient rats with extremely high plasma TSH levels although the nyctohemeral B rhythm was normal. Plasma TSH was approximatley twice as high in males (M) as in females (F) (overall mean .+-. SE) M = 149 .+-. 11, F = 81 .+-. 7 .mu.U/ml, P < 0.001. There was no significant difference (P > 0.05) in plasma TSH at different stages of the estrous cycle. Plasma T4 was slightly, but significantly, higher in males than females (overall mean .+-. SE: M = 6.4 .+-. 0.1, F= 6.0 .+-. 0.1 .mu.g/100 ml; P < 0.001), while T3 was higher in females than in males (overall mean .+-. SE: M = 69.5 .+-. 1.7, F = 80.3 .+-. 2.1 ng/100 ml; P < 0.001). No significant nyctohemeral rhythm was observed in plasma T4 or T3 in either sex. There is a nyctohemeral rhythm of plasma TSH which is independent of plasma B fluctuations and not associated with proportional changes in plasma thyroid hormones. A sustained high rate of TSH secretion abolishes the normal nyctochemeral plasma TSH rhythm. There are significant differences in plasma concentrations of TSH, T4 and T3 between male and female rats.