Background: Vitamin D insufficiency is very common in general population and in patients with primary hyperparathyroidism (PHPT). Measurement of serum 25OHD (25hydroxy-vitamin D) is currently the best available test to assess vitamin D adequacy. However, it has a significant seasonal variation, and is difficult to define a serum 25OHD level that is sufficient for bone health, although a serum 25OHD above 50 nmol/l has been suggested.
Previous studies confirm a significant negative correlation between 25OHD, and parathyroid hormone (PTH), alkaline phosphatase that is reversible. In the NHANES study the higher serum 25OHD then the higher the BMD throughout the reference range. The greatest impact of vitamin D on PTH is found when 25OHD is <50 nmol/l. PTH levels have been shown to decline until 25OHD above 75 nmol/l.
Aim: To determine if a single recorded vitamin D level above 50 nmol/l in routine clinic practice can safely exclude vitamin D deficient secondary hyperparathyroidism.
Study method: A retrospective study of 480 postmenopausal women who had high PTH levels during screening at an osteoporosis clinic was conducted.
Results: Fifty-two of 480 patients (10.8%) had PHPT, 19 (4%) had renal related secondary hyperparathyroidism, and 222 (46.25%) had 25OHD level <50 nmol/l. The remaining 170 who had normal renal function, normal adjusted calcium but 25OHD >50 nmol/l and a high PTH appeared to have normocalcaemic hyperparathyroidism. Six month later, with continued replacement of vitamin D and calcium, PTH was normalized in 65 patients, remained high with normal calcium in 55 patients and not checked in 47 patients. Three progressed to hypercalcaemic PHPT. 38 of 55 patients who remained normocalcaemic hyperparathyroidism showed 25OHD <50 nmol/l on repeat testing.
Conclusion: The 25OH-D level is variable and a single measurement may not exclude vitamin D insufficiency. The observation also suggested that many cases of normocalcaemic hyperparathyroidism might simply have or coexist with vitamin D insufficiency.