Purpose: Differences in the prevalence of vitamin D deficiency may explain why the frequency of symptoms in patients with primary hyperparathyroidism varies geographically. This study was performed to determine the prevalence in the United States of low 25-hydroxyvitamin D levels among patients with mild primary hyperparathyroidism, and the effect of 25-hydroxyvitamin D status on disease severity.
Methods: We studied 124 patients with mild primary hyperparathyroidism. Biochemical, bone mineral density, and bone histomorphometric indices were compared among patients whose serurm 25-hydroxyvitamin D levels were in the lowest and highest tertiles.
Results: Serum 25-hydroxyvitamin D levels (mean +/- SD) were in the low range of normal (21 +/- 11 ng/mL, normal 9 to 52 ng/mL). Levels were below normal in 9 (7%) patients, and below the level suggested for vitamin D "sufficiency" (20 ng/mL) in 66 (53%) patients. Those with lowest 25-hydroxyvitamin D levels had the highest parathyroid hormone levels (low tertile 158 + 66 pg/mL versus high tertile 103 +/- 2 pg/mL, P <0.0001). Other evidence of more active hyperparathyroidism in those with low 25-hydroxyvitamin D levels included higher serum alkaline phosphatase activity (114 +/- 48 U/L versus 91 +/-35 U/L, P <0.03), lower serum phosphorus levels (2.7 +/- 0.4 mg/dL, versus 3.0 +/- 0.4 mg/dL, P <0.01), and greater bone mineral density at the lumbar spine (0.94 +/- 0.03 g/cm2 versus 0.83 +/- 0.03 g/cm2, P <0.05) reflecting the protective effects of parathyroid hormone on cancellous bone. They also had enhanced bone turnover on bone biopsy. Despite the expected differences in vitamin D metabolism in African-Americans, results did not differ by race.
Conclusion: Vitamin D insufficiency or deficiency is common among patients with mild primary hyperparathyroidism. In these patients, the effects of primary hyperparathyroidism on biochemical, densitometric, and histomorphometric indices are more pronounced.