The Threshold of Vitamin D Deficiency should not be related to PTH
Maria Walsh, Donncha O'Gradaigh, Melanie Fox
Department of Rheumatology, UHW
The interaction between parathyroid hormone (PTH) levels, calcium levels and vitamin D is not fully explained. Threshold levels of vitamin D deficiency have been determined based on changes in PTH, and adverse effects on bone are thought to be mediated by PTH. However, both serum vitamin D levels and serum PTH measurements are usually taken during assessment of bone health.
To measure the correlation between PTH and vitamin D and to determine if a cut-off point of PTH measurement could be identified that would signify vitamin D deficiency / sufficiency without the need to run both tests.
A pragmatic sample group from the DXA and FLS Report database (2014-17) at Department of Rheumatology, UHW, which is filed by patient MRN. A sample of 230 reports were screened. Of these, 105 had measurements of both serum 25-hydroxy-vitamin D [vitamin D] and serum intact PTH [PTH] available (15 males (mean age=67) and 90 females (mean age=68)).
In total, 105 contemporaneous samples of vitamin D and PTH levels were analysed. A correlation of -0.3 between serum 25hydroxy vitamin D level and serum intact PTH was found. Mean vitamin D in those with normal PTH level (<65pg/ml) was 61 nmol/L. Mean vitamin D when PTH was raised (>65pg/ml) was 41nmol/L. As 36% of patients with normal PTH had vitamin D <50nmol/L (false negative) while 40% of those with raised PTH had vitamin D >50nmol/L (false positive), a cut-off value was not realistic and ROC analysis was not carried out. One case of hypercalcaemia was noted, and follow-up revealed a parathyroid adenoma. (This patient has declined surgery and remains medically well). Potential variation in PTH assay secondary to delay in transportation of samples from a wide geographical area was excluded in a sub-study with the UHW laboratory. A sample of those patients with discordant results were requested to present for repeat blood tests to exclude the possibility of a blunted parathyroid response to vitamin D supplementation, but too few complied to allow analysis.
The relationship between vitamin D, parathyroid hormone and calcium is complex, and the literature on effects on bone turnover suggests that the requirement for vitamin D intervention should be based on a threshold of 50nmol/L irrespective of parathyroid levels. Our finding of a poor correlation is consistent with the literature in other, non-Irish settings.