Pituitary-Thyroid Hormone Periodicities in Serially Sampled Plasma of Unanesthetized Rats*

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Abstract
Circadian and ultradian rhythms of plasma TSH, T3 and T4 were investigated in rats from which serial blood samples were obtained every 4 h for 3 days (low frequency) or every 15 min for 2–3 h (high frequency). Eleven male adult Sprague-Dawley rats, weighing 400–600 g, were maintained in individual cages at 28 C with artificial lighting from 0630–1830 h. Whole blood samples were drawn remotely via indwelling intraatrial catheters from unanesthetized animals enjoying normal activity, and the plasma concentrations of TSH, T3, and T4 were measured in triplicate by RIAs. Plasma corticosterone was also measured in the eight rats sampled at low frequency. Circadian periodicity was evident for plasma TSH, the mean concentration ranging from 138 ± 9 to 50 ± 4 (SE) μU/ml (n = 25). Eighteen of 25 diurnal cycles (eight different rats) had TSH peaks at 1100 or 1500 h; 16 had nadirs from 2300–0700 h. Circadian variation of plasma corticosterone was also seen in most rats, with the average peak amplitude 4–8 h in advance of TSH. Plasma T3 variation appeared periodic but inconsistent in pattern. Seven high frequency sampling experiments using 3 different rats demonstrated additional oscillations in plasma TSH with amplitudes ±100% of the mean and periods of 75-120 min. Ultradian periodicities of plasma T3 of lesser amplitude and frequency were also observed lagging 15–60 min behind changes in TSH. In summary, definite periodic fluctuations were observed inplasma corticosterone, TSH, and T3, but not T4. This study reveals the extent to which endogenous pituitary-thyroid function can vary within and among animals of the same stock living in a common controlled environment.