Changes in parathyroid hormone and body mass index and the association with mortality in dialysis patients

Background. Obesity is associated with secondary hyperparathyroidism in the general population. It is unknown whether body mass index (BMI) affects parathyroid hormone (PTH) level and its association with mortality in dialysis patients. Methods. From a prospective cohort study of incident dialysis patients in the Netherlands (NECOSAD), we selected all patients with recorded BMI and PTH at 3 months (baseline) after the start of dialysis [n = 1628, age 59 +/- 15 years, BMI 24.7 +/- 4.1 kg/m(2), median PTH 13.0 (interquartile range 5.3-29.0) pmol/L]. We assessed associations between BMI and PTH at baseline and between their changes over 3 months by correlation and linear regression analyses. The effect of the changes in PTH on all-cause mortality during a subsequent mean follow-up of 3.2 +/- 2 years was assessed by Cox regression analyses. Results. Median PTH levels at baseline were lowest in underweight patients (10.2 pmol/L), followed by normal weight (12.1 pmol/L), overweight (14.0 pmol/L) and obese patients (17.5 pmol/L). The associations were similar in diabetic and non-diabetic patients. A >= 5% decrease in BMI (n = 101) over 3 months was accompanied by a 26% decrease in PTH (PTH(ratio) 0.74; P = 0.039), whereas a >= 5% increase in BMI (n = 143) was associated with an 11% increase in PTH (PTH(ratio) 1.11; P = 0.026). Compared to patients with stable PTH levels, patients with decreasing PTH in the presence of weight loss showed a 2-fold higher mortality (hazard ratio 2.02, 95% confidence interval 1.45-2.83; P < 0.001), in contrast to those with decreasing PTH in the absence of weight loss. Additional analyses showed that the weight loss was responsible for increased deaths. Conclusions. PTH is associated with BMI and its longitudinal changes in dialysis patients, both in patients with and without diabetes mellitus. An increased mortality seen for patients with concurrent decreases in PTH and BMI was explained by the weight loss, representing an important confounder for outcome analyses according to levels of PTH. Low and decreasing PTH levels may be symptoms of wasting, which should be taken into account in the care of dialysis patients.