The relationship between serum 25(OH)D and parathyroid hormone levels.

OBJECTIVE

Low 25(OH)D levels are associated with increased parathyroid hormone levels leading to progressive bone loss. The serum levels of 25(OH)D sufficient to keep the parathyroid hormone level at a range that will prevent bone loss are still unclear. The current study was aimed at evaluating the relationship between 25(OH)D levels and concomitant parathyroid hormone levels.

METHODS

The computerized laboratory database of Clalit Health Services, a not-for-profit health maintenance organization covering more than half of the Israeli population, was searched for all 25(OH)D and parathyroid hormone tests performed in 2009. Concomitant tests of parathyroid hormone and 25(OH)D were identified in 19,172 people.

RESULTS

Serum parathyroid hormone levels were inversely correlated with 25(OH)D levels (r = -0.176, P < .001); 25(OH)D levels less than 50 nmol/L were associated with a steep increase in parathyroid hormone levels and hyperparathyroidism, which decreased with increasing 25(OH)D levels and reached a plateau at 25(OH)D levels of 75 to 85 nmol/L. The quadratic fit with plateau model showed that parathyroid hormone stabilizes at 25(OH)D level of 78.9 nmol/L. However, after excluding 5449 people with hypercalcemia or renal failure, the parathyroid hormone plateau was attained at a significantly lower 25(OH)D cut point of 46.2 nmol/L.

CONCLUSION

Our data suggest that a 25(OH)D threshold of 50 nmol/L is sufficient for parathyroid hormone suppression and prevention of secondary hyperparathyroidism in persons with normal renal function. 25(OH)D levels greater than 75 nmol/L do not seem to be associated with additional change in parathyroid hormone levels.