Parathyroid hormone is not involved in 1,25-dihydroxyvitamin D regulation of 25-hydroxyvitamin D metabolism in vivo

1,25-Dihydroxyvitamin D₃ administration to vitamin D-deficient rats suppresses accumulation of 1,25-dihydroxy-3α-³H]vitamin D₃ and stimulates accumulation of 24,25-dihydroxy-3α-³3H]vitamin D₃ from 25-hydroxy-3α-³H]vitamin D₃ equally well in the presence and absence of parathyroid glands. These results demonstrate that this regulatory action is not mediated by the parathyroid glands and support conclusions from $\text{in}\text{vitro}$ studies that this represents a direct action of 1,25-dihydroxyvitamin D₃.