Influence of Thyroid Function on the Serum Concentration of 1,25-Dihydroxyvitamin D3*

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Abstract
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/l, (mean .+-. SD); n = 23; P < 0.01] and increased in hypothyroidism (53 .+-. 17 mU/l, n = 12; P < 0.001) compared to that in normal subjects (26 .+-. 9 mU/l, 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/l) than in the serum of normal subjects (42 .+-. 13 ng/l). An increased concentration of 1,25-dihydroxyvitamin D3 was observed in the serum of hypothyroid patients (73 .+-. 28 ng/l, 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 Ca absorption in thyroid disorders.