DECREASED BONE SENSITIVITY OF THYROIDECTOMIZED RATS TO THE CALCAEMIC EFFECT OF 1,25-DIHYDROXYCHOLECALCIFEROL

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Abstract
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 thyroidintact parathyroid transplanted rats thyroidectomized animals had a significantly reduced calcaemic response to 1,25-dihydroxycholecalciferol, Daily supplementation with d,l-thyroxine (100 μg/rat) during the experimental period restored a normal response. The increase in serum calcium concentration after 1,25-dihydroxycholecalciferol injection was 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.