Effects of vitamin D3 metabolites on thyrotropin secretion from rat pituitary cells in culture.

The effects of several vitamin D3 metabolites on TSH secretion were investigated using rat pituitary cells in culture. After a 24-h incubation in Dulbecco's medium, only 25,26-dihydroxyvitamin D3 [25,26(OH)2D3] at 10(-6) M increased the basal TSH secretion (36% above control). The TRH-induced TSH secretion increased dose-dependently with increasing concentrations of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] showing a significant increase at 10(-8) M (41% above control). At 10(-5) M 25,26(OH)2D3 the TRH-stimulated TSH secretion was also increased (63% above control). In medium containing charcoal-treated serum, 1, 24,25-trihydroxyvitamin D3 [1,24,25(OH)3D3] decreased the TRH-induced TSH secretion dose dependently (34% below control at 10(-8) M), while the effects of both 1,25(OH)2D3 and 25,26(OH)2D3 were blunted. Our results indicate that several vitamin D3 metabolites may affect pituitary TSH secretion.