Influence of thyroid function on the serum concentration of 1,25-dihydroxyvitamin D3

The serum concentrations of parathyroid hormone, 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 were measured in patients with untreated thyroid disorders. The serum concentration of parathyroid hormone was decreased in hyperthyroidism [20 +/- 10 mU/liter, (mean +/- SD); n =23; P < 0.01] and increased in hypothyroidism (53 +/- 17 mU/liter; n = 12; P < 0.001) compared to that in normal subjects (26 +/- 9 mU/liter; n = 81). The concentration of 25-hydroxyvitamin D3 was not altered, but the concentration of 1,25-dihydroxyvitamin D3 was significantly lower in the serum of hyperthyroid patients (28 +/- 11 ng/liter) than in the serum of normal subjects (42 +/- 13 ng/liter). On the contrary, an increased concentration of 1,25-dihydroxyvitamin D3 was observed in the serum of hypothyroid patients (73 +/- 28 ng/liter; P < 0.001 vs. normal subjects). The abnormal serum concentrations of 1,25-dihydroxyvitamin D3 in thyroid disorders cannot be explained by differences in serum binding because the serum vitamin D-binding protein was unaltered in hyperthyroid subjects and only slightly increased (+17%) in hypothyroid subjects. These changes in the serum concentration of 1,25-dihydroxyvitamin D3 are compatible with previous data on altered intestinal calcium absorption in thyroid disorders.