Influence of age, strain and season on diurnal periodicity of thyroid stimulating hormone, thyroxine, triiodothyronine and parathyroid hormone in the serum of male laboratory rats.

The influence of age, strain and season on the diurnal pattern of serum hormone levels from the pituitary-thyro-parathyroid complex was studied in male laboratory rats. Distinct 24 h periodicity in the serum levels of thyroid stimulating hormone (TSH) and triiodothyronine (T3) was observed in all groups of rats. There was no influence of age (40, 60 and 90 days old Sprague-Dawley rats), but a significant influence of strain (Sprague-Dawley vs. BH/Ztm rats) and season (summer vs. winter) on the diurnal pattern of serum TSH and T3 levels. Significant 24 h periodicity in serum thyroxine (T4) levels existed during winter in BH/Ztm rats, but not in Sprague-Dawley (SD) rats of any age. Adult SD rats demonstrated 24 h periodicity in serum levels of T4 only in summer. No diurnal periodicity in serum levels of parathyroid hormone (PTH) was observed in any group of rats. There were significant changes in 24 h mean serum levels of TSH and T3 throughout pubertal development. Twenty-four h mean serum levels of T3 and T4 were significantly higher in summer than in winter. Twenty-four h mean serum levels of T4 were significantly lower in BH/Ztm rats than in SD rats. Significant correlation was observed between serum concentrations of T3 and T4, TSH and T4, and between TSH and T3 in some groups of rats, but not in all. The results indicate that 24 h periodicity of serum hormone levels from the pituitary-thyroid complex of male laboratory rats may vary with age and strain of the animals and with the season of experiment performance.