Effect of thyroparathyroidectomy of calcium metabolism in rats: role of 1,25-dihydroxyvitamin D3.

Thyroparathyroidectomy (TPTX) decreases plasma calcium, bone formation and resorption, and tubular reabsorption of calcium. It also reduces the production of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] which very likely results in the decrease in the intestinal calcium absorption (Vna) observed after TPTX. We have examined whether the influence of TPTX on plasma calcium and bone calcium fluxes could be corrected by doses of 1,25-(OH)2D3 (2 X 13 pmol/day ip), which just normalize Vna. The study was made by calcium balance and 45Ca kinetics in rats receiving a normal supply of vitamin D3. The results show that in TPTX rats physiological doses of 1,25-(OH)2D3 increased plasma calcium and decreased plasma phosphate. Calcium retention was not fully corrected because 1,25-(OH)2D3 increased urinary calcium excretion. The deposition into and the release of calcium from bone were enhanced by 1,25-(OH)2D3 but remained lower than in pair-fed sham-operated animals. Thus, in as much as the diminished renal production of 1,25-(OH)2D3 in TPTX is entirely responsible for the reduced Vna, the decrease in 1,25-(OH)2D3 could only partly explain the effect of thyroparathyroidectomy on bone calcium fluxes.