Seasonal variance in serum levels of vitamin d determines a compensatory response by parathyroid hormone: study in an ambulatory elderly population in Quebec.

BACKGROUND Although the seasonal variance in serum levels of vitamin D in the elderly is well known, its significance on parathyroid hormone (PTH) remains controversial. OBJECTIVE To identify the variability and correlation between serum levels of vitamin D and PTH in a sample of community-dwelling elderly patients in the Province of Quebec, Canada, where vitamin D and calcium are supplemented in the food. METHODS Cross-sectional study in an ambulatory elderly population in the Province of Quebec. Samples were analyzed at the Metabolic and Calcium Research Centre, Royal Victoria Hospital, Montreal, Quebec. 256 healthy men and women aged 65-94 (mean age +/- SD: 72.8 +/- 5.6) were analyzed. Serum levels of 25-hydroxyvitamin D (25(OH)D3) and PTH were determined between 1994 and 1999 using commercial radioimmunoassay kits to measure calciotropic hormones. We examined data in different seasons of the year and observed the behavior of the data through time. A cut-off level of 25 nmol/l for 25(OH)D3 was established to define vitamin D deficiency. A correlation between vitamin D levels vs. PTH levels was also obtained. RESULTS There is a predominance of females with a 75% of the population. Among them, 32% showed levels of vitamin D <20 nmol/l as compared to 51% of the male population (p < 0.02). A seasonal variance in the levels of vitamin D was observed with the lower levels happening in early spring with a recovery at the end of the summer (p < 0.004). These low levels of vitamin D corresponded with an inverse pattern in the levels of PTH more importantly in early spring. CONCLUSION This study not only confirms previous reports that despite vitamin D food supplementation a vitamin D deficiency is still a finding in elderly population in the Northern hemisphere, but also that a compensatory change in PTH levels concurrently occurs with a potential significance on bone strength and risk of fractures.