Twelve hypothyroid subjects, 13 healthy and 12 healthy women with a slight deficiency
of vitamin D were studied to distinguish seasonal changes from the thyroxine-dependent
ones. Serum 25-hydroxyvitamin D levels of hypothyroid patients were lower than those
of healthy individuals when the sera were obtained in the autumn. In hypothyroid patients
a single oral dose of 100,000 IU vitamin D3 resulted in a smaller increase in 25-hydroxyvitamin
D concentration than in controls having subclinical exogenous vitamin D deficiency.
Substitution therapy with thyroid hormone, started in our study always in autumn,
increased the 25-hydroxyvitamin D concentration in hypothyroid patients, which was
opposite to the autumn-to-spring variation of this hormone observed in healthy controls.
The increase of 25-hydroxyvitamin D, dehydroepiandrosterone and its sulphate values
following substitution therapy in the hypothyroid patients may indicate that thyroid
hormone(s) is (are) involved in the regulation of steroid hormone synthesis.