Effect of short-term hyperthyroxinemia on vitamin D metabolism in congenital hypothyroidism

The circulating concentrations of vitamin D metabolites were measured in nine children (four to ten years of age) with congenital hypothyroidism on L-thyroxine therapy, before and after a short term increase (33%) in dosage. The concentrations of 25-hydroxy vitamin D and 24,25-dihydroxy vitamin D were not altered, but the concentration of 1,25 dihydroxyvitamin D was significantly higher in the serum of the children after three weeks of hyperthyroxinemia. This was associated with an increase in urinary calcium excretion. The increases in serum concentration of 1,25 dihydroxy vitamin D cannot be explained by differences in serum levels of calcium, phosphorus or parathyroid hormone. These findings differ from data obtained in adults.