Defining vitamin D status by secondary hyperparathyroidism in the U.S. population
A.A. Ginde1, P. Wolfe2, C.A. Camargo Jr3, and R.S. Schwartz4
1Department of Emergency Medicine, University of Colorado Denver School of Medicine; 2Department of Biostatistics and Informatics, Colorado School of Public Health, Aurora, CO; 3Department of Emergency Medicine, Massachusetts General Hospital, Harvard Medical School, Boston, MA; 4Division of Geriatric Medicine, University of Colorado Denver School of Medicine, Aurora, CO, USA

Background: Vitamin D is associated with a variety of health outcomes, but the exact definition of vitamin D sufficiency remains controversial. Aim: We sought to define skeletal-related vitamin D sufficiency by estimating maximum PTH suppression in the U.S. population. Methods: We performed a cross-sectional analysis of the National Health and Nutrition Examination Survey (NHANES), 2003-2006. We examined the association between serum 25-hydroxyvitamin D (25OHD) level and serum PTH level in 14,681 participants aged ≥6 yr. We also evaluated the 25OHD-PTH association using 2 thresholds of hyperparathyroidism: PTH≥45 pg/ml and ≥75 pg/ml. Results: The mean 25OHD level was 24 ng/ml and mean PTH was 42 pg/ml. PTH≥45 pg/ml was present in 35% of the population, while PTH≥75 pg/ml was present in 7%. The prevalence of 25OHD levels <40 ng/ml and <30 ng/ml was 95% and 77%, respectively. In both unadjusted and adjusted models, there was a strong inverse relationship between 25OHD and PTH. Compared to 25OHD≥40 ng/ml, the 25OHD-PTH association was 2.36 [95% confidence interval (CI), 2.08-2.67] times greater for 25OHD<5 ng/ml and 1.12 (95%CI, 1.07-1.17) times greater for 25OHD 30-39.9 ng/ml. Compared to 25OHD≥40 ng/ml, 25OHD levels of 20-29.9 ng/ml [odds ratio (OR) 2.0 (95%CI, 1.4-2.8)] but not 30-39.9 ng/ml [OR 1.1 (95%CI, 0.8-1.6)] were independently associated with PTH≥45 pg/ml. Conclusions: Optimal vitamin D status, defined by estimated maximum PTH suppression, does not occur until at least 25OHD levels ≥40 ng/ml. Using these thresholds, most of the U.S. population needs more vitamin D. Large, prospective studies are needed to determine optimal vitamin D supplementation. (J. Endocrinol. Invest. 35: 42-48, 2012) ©2012, Editrice Kurtis

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