Endocrine Abstracts

Introduction: Vitamin D insufficiency and deficiency are defined as a serum 25-hydroxy-vitamin D (25(OH)D) below 50 and 30 nmol/l respectively. We aimed to determine whether there is a serum 25(OH)D that signals a low serum calcium and phosphate, secondary hyperparathyroidism, high bone remodelling, low area bone mineral density (aBMD), and so, an increased risk for microstructural deterioration and bone fragility.

Method: Concentrations of 25(OH)D, calcium, phosphate, creatinine and parathyroid hormone (PTH) were measured by Melbourne Pathology in serum from 11,855 subjects (8777 women, 3078 men). We measured serum C-terminal telopeptide of type 1 collagen (CTX), Procollagen type 1 N-terminal propeptide (P1NP) and aBMD at the spine and hip in 182 subjects from Austin Health. We excluded persons <20 years, patients with hypercalcaemia, chronic kidney disease and a serum 25(OH)D ≥180 nmol/l.

Results: Serum calcium and phosphate correlated positively with serum 25(OH)D. Serum PTH and alkaline phosphatase, but not CTX and P1NP, correlated negatively with serum 25(OH)D. There was no detectable association between serum 25(OH)D and aBMD and no level of 25(OH)D that identified persons with low serum calcium or phosphate, or high PTH or remodelling markers. Among 1439 subjects with 25(OH)D <30 nmol/l, 6.1% had a low serum calcium, 3.4% had a low serum phosphate, 6.1% had a high alkaline phosphatase, and 34.2% had an elevated PTH.

Conclusion: The sample size of subjects with 25(OH)D below 30 nmol/l may have limited the power to detect associations, but within this constraint and the cross sectional nature of this study, we infer that diagnosing persons as having vitamin D ‘deficiency’ or ‘insufficiency’ based on the current criteria is not evidence based. A diagnostic threshold level of serum 25(OH)D predisposing to bone disease, and the duration of exposure at this level, remain uncertain.