Evidence for Circadian Variations in Serum Thyrotropin, 3,5,3′-Triiodothyronine, and Thyroxine in the Rat*

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Abstract
We previously observed that under a 12-h light, 12-h dark schedule (lights off at 1900 h), male Sprague-Dawley rats showed a circadian rhythm for serum TSH with a zenith near midday. In the present work we further characterized the serum TSH rhythm by appropriate mathematical analysis. The peak of serum TSH occurred at 1130 ± 0105 h (mean of five experiments), with an amplitude of 0.2 ±0.1 μg/ml, while the TSH level was minimum at the beginning of the dark period; the period of the TSH rhythm was 24.5 ± 0.6 h. A circadian rhythm was also demonstrated for serum T3 and T4; these rhythms were characterized by peaks occurring 1.5 and 2.2 h after that of TSH, respectively. As expected, characteristics of the rhythm were more narrowly defined for T3 than for T4. Serum concentrations peaked at 1256 ± 0206 and 1346 ± 0308 h for T3 and T4, respectively; the serum T3 concentration varied from 78. ± 7.8 to 54.2 ± 2.7 ng/100 ml (P < 0.001) and serum T4 varied from 7.3 ± 1.1 to 5.1 ± 0.5 μg/100 ml (P < 0.05) for zenith and nadir values, respectively. It is suggested that the diurnal peaks of thyroid hormones might be related to TSH-induced changes in thyroid secretion.

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