Decreased bone sensitivity of thyroidectomized rats to the calcaemic effect of 1,25-dihydroxycholecalciferol

The calcaemic response of thyroidectomized parathyroid transplanted rats to a single dose of biosynthetic 1,25-dihydroxycholecalciferol (50 ng) injected into a jugular vein, was evaluated. The animals were fed a vitamin D-free, low calcium diet. Compared to sham-operated and to thyroid-intact parathyroid transplanted rats thyroidectomized animals had a significantly reduced calcaemic response to 1,25-dihydroxycholecalciferol. Daily supplementation with d,1-thyroxine (100 mug/rat) during the experimental period restored a normal response. The increase in serum calcium concentration after 1,25-dihydroxycholecalciferol injection was a similar in thyroidectomized bilaterally nephrectomized animals, and in thyroidectomized kidney-intact rats. The results suggest that in thyroxine depleted rats, the sensitivity of bone to the calcaemic effect of 1,25-dihydroxycholecalciferol is decreased.