Serum parathyroid hormone level is associated with body mass index. The 5th Tromso study

Objective: To study whether serum parathyroid hormone (PTH) and serum calcium are associated with body mass index (BMI), and their predicting role in obesity. Design: Population based, cross-sectional study. Methods: In 2001 a population-based health survey was held in Tromso, North Norway. Questionnaires on medical history and life-style factors were completed and anthropometric data were collected. Calcium and vitamin D intakes and a physical activity score were calculated. Serum calcium and PTH were measured in a subset of 3447 men and 4507 women. Pearson correlation and linear regression were used to evaluate associations between BMI, PTH and serum calcium, and logistic regression was used to test PTH and serum calcium as predictors of obesity and to calculate odds ratio. Relative risk was calculated using frequency tables. Results: For serum calcium and PTH there was a significant positive relation to BMI in both genders (P < 0.001), which to our knowledge has not previously been reported on the basis of a large epidemiological study. Age, low calcium and vitamin D intakes were explanatory variables for serum PTH. The highest quartile of serum PTH (> 4.20 mumol/l) was a significant predictor for obesity (P < 0.001) in both genders, adjusted for age, physical activity and serum calcium. Obesity rates were higher in those with PTH levels in the highest quartile compared with those in the lower quartiles, which resulted in a relative risk of 1.40 (95% confidence interval (C.I.) 1.20-1.60) for men and 1.48 (95% C.I. 1.31-1.67) for women. Conclusions: Serum PTH, adjusted for age, physical activity and serum calcium, is positively associated with BMI in both sexes, and serum PTH is an independent predictor of obesity in our statistical model.