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.
