Relationship between vitamin D, calcium and parathyroid hormone in Cape Town

Aim: The aim of this study was to test the hypothesis that vitamin D deficiency is associated with abnormal levels of calcium and parathyroid hormone (PTH). Methods: Vitamin D requests at a tertiary hospital in South Africa over 2 years were retrospectively analysed along with calcium and PTH levels. Results: Only when the 25-hydroxyvitamin D (25(OH) D) level dropped below 25 nmol/l, was there a significant rise in PTH. A subnormal 25(OH) D level was also not always related to hypocalcaemia, as more than half of patients with their 25(OH) D level below 25 nmol/l had calcium levels in the reference range. However, all patients with calcium levels below 1.8 mmol/l were shown to have vitamin D insufficiency. Conclusion: Hypovitaminosis D may co-exist with a blunted PTH response. Therefore, assumptions about vitamin D status should not be made based on PTH and calcium values. 25(OH) D measurements should be requested when vitamin D deficiency is clinically suspected, irrespective of biochemical results.