Seasonal variation of serum 25-hydroxyvitamin D in patients with chronic renal failure treated by regular haemodialysis.

Serum 25-hydroxyvitamin D was measured by a competitive protein-binding assay in 44 normal subjects, 60 uraemic patients on regular haemodialysis at different times of the year and in 13 non-dialyzed uraemic patients. The results obtained indicate that uraemic patients on regular haemodialysis have a mean serum 25-hydroxyvitamin D concentration comparable to controls and that they also exhibit a seasonal variation with a significant reduction during the winter months. However, serum 25-hydroxyvitamin D concentration remained essentially within the normal range and did not reflect the increased incidence of osteomalacia in these patients. In the 13 non-dialyzed uraemic patients, serum 25-hydroxyvitamin D concentrations were lower than in the dialyzed patients, but the explanation is not yet clear. This reduction in serum 25-hydroxyvitamin D was not accompanied by any osteomalacia. The results indicate that deficiency of 25-hydroxyvitamin D in our patients on regular haemodialysis is uncommon and clearly not the explanation of dialysis osteomalacia.