Serum PTH is not a good marker for defining a threshold for vitamin D deficiency
Methods: 2803 subjects from a general health survey, the Tromsø study, and pooled individual person data from five vitamin D intervention studies (n = 1544) were included. Serum parathyroid hormone (PTH) and change in PTH after vitamin D supplementation was related to serum 25(OH)D levels in steps of 25 nmol/L (<24, 25-49, 50-74, 75-99, >99 nmol/L).
Results: In the Tromsø study there was in the females a gradual decrease in serum PTH with increasing serum 25(OH)D with no apparent plateau, whereas in the males the decrease in PTH in subjects with serum 25(OH)D >74 nmol/l was marginal. In pooled RCTs, there was a significant reduction in serum PTH by vitamin D supplementation regardless of baseline serum 25(OH)D level.
Conclusions: The use of the serum PTH – 25(OH)D relation from observations studies to determine a threshold for vitamin D sufficiency is highly questionable.