Background: Vitamin D deficiency is a common feature of primary hyperparathyroidism (PHP). However, replacement of vitamin D is not routinely performed in clinical practice, due to concerns about its safety. Furthermore, it is not clear whether vitamin D deficiency worsens the biochemical derangements observed in patients with PHP.
Aim: To determine the relationship between 25(OH) vitamin D status and biochemical markers in patients with PHP.
Methods: We conducted a large retrospective study of 1013 biochemical measurements from 251 patients diagnosed with PHP using clinical, biochemical, radiological and histological criteria. Biochemical measurements for plasma parathyroid hormone (PTH), serum phosphate, albumin-adjusted serum calcium and 24-hour total urine calcium excretion, and calculated values for urine calcium-to-creatinine ratio (UCCR) were collated. We excluded patients with renal failure, or those on thiazides or lithium.
Results: In patients with PHP, serum 25(OH)D was correlated negatively with plasma PTH (r=−0.17, P<0.0001), and correlated positively with serum phosphate (r=0.13, P<0.0001) and urine calcium (r=0.18, P<0.001). PTH hypersecretion was most severe in PHP patients with vitamin D deficiency (mean plasma PTH in pmol/L: 18.9±0.7, 25(OH)D<25 nmol/L; 15.0±0.4, 25(OH)D>25 nmol/L; P<0.001). Phosphate wasting was most severe in PHP patients with vitamin D deficiency (mean serum phosphate in mmol/L: 0.86±0.01, serum 25(OH)D<25 nmol/L; 0.91±0.006, serum 25(OH)D>25 nmol/L; P<0.001). Vitamin D deficiency was associated with significantly lower UCCR in patients with PHP (mean UCCR: 0.0181±0.0008, 25(OH)D<25 nmol/L; 0.0216±0.0006, 25(OH)D>25 nmol/L; P<0.001). Mean hypercalcaemia was unaltered by serum 25(OH) vitamin D deficiency (mean serum calcium in mmol/L: 2.71±0.01, 25(OH)D<25 nmol/L; 2.70±0.006, 25(OH)D>25 nmol/L; P=0.26).
Conclusion: Our data suggest that in patients with PHP, vitamin D deficiency is associated with more severe derangements in PTH and phosphate levels, but no significant changes in mean serum calcium levels. Furthermore, vitamin D deficiency reduces the diagnostic accuracy of UCCR. We recommend that vitamin D replacement should be considered in patients with PHP and coexistent vitamin D deficiency.
Declaration of interest: There is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.
Funding: No specific grant from any funding agency in the public, commercial or not-for-profit sector.