Nyctohemeral and sex-related variations in plasma thyrotropin, thyroxine, and triiodothyronine

Abstract

TSH, T4and T3 were measured by radioimmunoassay in plasma samples obtained from 77 young adult male and 114 female rats fed a Purina high-iodine diet and maintained in an isolated room, 2-4/cage, at 24 ± 1 C with light from 0600-1800 h. In one experiment, 7 male and 7 female rats were decapitated every 3 h for 30 consecutive h and bunk 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 180Ã out of phase and negatively correlated (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 μ/100 ml; P < 0.001), while T, was higher in females than in males (overall mean ± SE: M = 69.5 ± 1.7, F = 80.3 ± 2.1 μ/100 ml; P < 0.001). No significant nyctohemeral rhythm was observed in plasma T4 or T3 in eith r sex. These observations indicate that: 1) There is a nyctohemeral rhythm of plasma TSH which is independent of plasma B fluctuations and not associated with proportional chaμes in plasma thyroid hormones. 2) A sustained high rate of TSH secretion abolishes the normal nyctohemeral plasma TSH rhythm. 3) There are significant differences in plasma concentrations of TSH, T4 an d T3. © 1975 by The Endocrine Society.