The concentrations of 1,25-dihydroxyvitamin D [1,25-(OH)2D], calcium, and phosphorus were measured in the serum of rats during pregnancy and at various stages of lactation. The concentration of 1,25-(OH)2D hormone increased almost two-fold during pregnancy and the latter part of lactation, but decreased to control levels or very low values immediately after birth and weaning, respectively. Furthermore, the concentration of 1,25-(OH)2D was inversely correlated with the concentration of calcium, suggesting that circulating 1,25-(OH)2D fluctuates in concert with calcium demands during the reproductive cycle. Parathyroidectomy in lactating rats caused a 70 percent inhibition of the normally observed 1,25-(OH)2D increase, indicating that parathyroid hormone, in response to changes in serum calcium, is a primary modulator of 1,25-(OH)2D during lactation.

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