Diurnal Variations of Plasma Thyrotropin, Thyroxine, and Triiodothyronine in Female Rats Are Phase Shifted after Inversion of the Photoperiod*

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Abstract
The present study examined plasma TSH, T4, and T3 concentrations in rats throughout the day to determine if diurnal variations of these hormones occurred. Female rats on a 12-h light, 12-h dark cycle (onset of light, 0730 h) were sampled by cardiac puncture at 2-h intervals throughout the day on 2 days, 1 week apart. Significant diurnal variations of plasma TSH, T4) and T3 were detected (P < 0.01). Peak TSH concentrations occurred soon after the onset of light, whereas T4 and T3 concentrations peaked 3–4 h later. After these variations were detected, the effect of inverting the photoperiod was examined. Female rats were placed on 12- h light, 12-h dark cycles, with the onset of light at 0730 h (LD) or 1930 h (DL). After 3 weeks, rats from each group were killed by decapitation at 4-h intervals throughout the day, and trunk blood was collected. Diurnal variations in plasma TSH, T4, and T3 (P < 0.01) were similar to those found with 2-h sampling intervals in the previous experiment, and plasma corticosterone and PRL rhythms peaked near the onset of darkness (P < 0.05). Inversion of the photoperiod phase shifted all diurnal variations, such that they maintained a similar phase relationship to the light-dark cycle on both LD and DL photoperiods. These results indicated that diurnal variations of plasma TSH, T4, and T3 concentrations could be repeatedly detected with different sampling protocols. These variations were phase shifted by inverting the photoperiod, which indicated that some aspect of the light-dark cycle can act to set the phase of these diurnal variations in the pituitary-thyroid axis.