Systemic effects of 1,25-dihydroxyvitamin D3 on the pituitary-hypothalamic axis in rats

Treatment of rats with 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) 0.05 microgram/kg per day for three days was without any effect on serum T3, T4 or TSH concentrations, whereas serum PRL increased (20.6 +/- 3.8 to 76.2 +/- 19.1 micrograms/l, mean +/- SEM, N = 7-8; P less than 0.01). Increased hypothalamic TRH levels (24.3 +/- 3.9 to 45.7 +/- 7.8 pmol/g wet weight; P less than 0.01) may indicate an effect of 1,25(OH)2D3 on hypothalamic TRH homeostasis. This effect could probably be due to an indirect action of 1,25(OH)2D3, mediated by the increased serum calcium (2.77 +/- 0.02 to 3.16 +/- 0.08 mmol/l, mean +/- SEM, N = 7-8; P less than 0.001). This assumption was, however, not tested. Neither the pituitary TSH nor PRL was affected. The treatment decreased the serum concentration of 25-hydroxyvitamin D3 (23.0 +/- 1.3 to 16.8 +/- 2.0 nmol/l, mean +/- SEM, N = 5-7; P less than 0.01) and of 24,25-dihydroxyvitamin D3 (3.2 +/- 0.3 to 2.1 +/- 0.1 nmol/l, mean +/- SEM, N = 3-5; P less than 0.05). The results show that in this experimental design, 1,25(OH)2D3 has no effect on basal hormone secretion from the pituitary-thyroid axis, and that 1,25(OH)2D3 decreases the synthesis of the vitamin D3 metabolites studied.