Free and bioavailable 25-hydroxyvitamin D levels in patients with primary hyperparathyroidism

Objective: To evaluate free and bioavailable 25-hydroxyvitamin D (25[OH]D) levels in primary hyperparathyroidism (PHPT) patients. Methods: Fifty PHPT patients and 50 healthy age-, gender-, and body mass index (BMI)-matched control subjects were enrolled. Levels of 25(OH)D were determined by a radioimmunoassay and vitamin D-binding protein (DBP) were determined by an enzyme-linked immunosorbent assay. Free and bioavailable 25(OH)D were calculated utilizing equations that use average binding coefficients for DBP and albumin. Results: There was no significant difference in age and BMI between PHPT patients and controls (P>.05). Levels of 25(OH)D, DBP, and DBP-bound 25(OH)D were lower in PHPT patients compared to controls (P<.01). There was no significant difference in free and bioavailable 25(OH)D levels between PHPT patients and controls (P>.05). Levels of intact parathyroid hormone were inversely correlated with free (r = -0.217; P<.05) and bioavailable 25(OH)D levels (r = -0.296; P<.01). Conclusion: Serum total 25(OH)D levels were lower, while free and bioavailable 25(OH)D remained similar in patients with PHPT compared to controls. We suggest that low 25(OH)D levels might not reflect true vitamin D nutrition status in PHPT patients.