Diurnal Rhythm of Pituitary-Thyroid Axis in Male Rats and the Effect of Adrenalectomy

Abstract

Levels of TSH, thyroxine (T₄) and triiodothyronine (T₃) were measured by radioimmunoassay in plasma (TSH, T₄, T₃) and pituitary (TSH) of 60-day old male Long-Evans rats. Definite diurnal rhythms were demonstrated in pituitary TSH, plasma TSH and T₃ in intact rats, evidenced by the statistically significant differences between zenith and nadir for pituitary TSH, plasma TSH and T₃. The zenith value of pituitary TSH and the nadir values of plasma TSH, T₄ and T₃ were observed at the same time (2400 h) as were the nadir value of pituitary TSH and the zenith values of plasma TSH, T₄, and T₃ (at 1200 h). Our results indicate that the rhythmicity of pituitary TSH content is a mirror image of that of plasma TSH. Adrenalectomy not only reduced plasma corticosterone levels to almost zero, but also decreased plasma T₃ and T₄, levels. In adrenalectomized rats, the absolute concentration of pituitary and plasma TSH increased at 1200 h and at 2400 h and, in both cases, the difference between values at 1200 h and at 2400 h persisted. The differences in plasma TSH, T₃ and T₄ between 1200 h and 2400 h were also observed in sham-aarenalectomized rats. These results suggest glucocorticoids do influence the pituitary-thyroid axis, but that the rhythmicity of the pituitary-thyroid axis does not seem to depend on the rhythmicity of glucocorticoid secretion.