Objective: To determine the serum level of free 1,25-dihydroxyvitamin D (1,25-[OH]2D) in patients with vitamin D toxicity and to assess the in vitro effect of differing concentrations of vitamin D metabolites on the free serum levels of 1,25-(OH)2D.
Design: 1] A case study of patients hospitalized with vitamin D toxicity after accidentally ingesting a veterinary vitamin D concentrate and 2) an in vitro experiment in which vitamin D metabolites in various concentrations were added to normal serum and their effect was noted on percentage of free 1,25-(OH)2D.
Patients: 11 patients (age range, 8 to 69 years) were studied 10 to 40 days after hospitalization for hypercalcemia.
Measurements: Serum total 25-hydroxyvitamin D (25-OHD) and 1,25-(OH) (2D) levels were measured by radioreceptor assays. The percentage of free 1,25-(OH)2D was measured by centrifugal ultrafiltration isodialysis and was used to calculate actual free 1,25-(OH)2D levels. In the in vitro studies, vitamin D metabolites (25-OHD; 24,25-[OH]2D; 25,26-[OH]2D; and 25-OHD-26,23 lactone) were added to normal serum in concentrations expected to occur with vitamin D toxicity. The percentage of free 1,25-(OH) (2D) was measured by isodialysis.
Results: All patients presented with marked hypercalcemia (mean calcium level, 3.99 ±0.33 mmol/L). Serum 25-OHD levels ranged from 847 to 1652 nmol/L, and total 1,25-(OH)2D levels (mean, 106 ±86 pmol/L) were elevated in only three patients. The percentage of free 1,25-(OH)2D (mean, 1.023% ±0.366%) was elevated in all nine patients in whom it was measured. Actual free 1,25-(OH)2D levels (mean, 856 ±600 fmol/L) were elevated in six of the nine patients. Total 1,25-(OH)2D levels were correlated with 25-OHD levels (r = 0.66; P = 0.03), whereas total and free 1,25-(OH)2D levels were highly correlated (r = 0.957; P < 0.001). In the in vitro studies, the percentage of free 1,25-(OH)2D increased after 25-OHD or 24,25-(OH)2D was added.
Conclusions: Although the patients had normal or near-normal total 1,25-(OH)2D values, most patients had elevated free 1,25-(OH)2D levels. These findings suggest that elevated free 1,25-(OH)2D levels might play a role in the pathogenesis of hypercalcemia in vitamin D toxicity.