Background: There is a direct relationship between age and serum parathyroid hormone (iPTH) in the normal population, but several studies suggest this relationship is reversed in maintenance hemodialysis (MHD) patients. The pathophysiologic basis of this age-related decline in serum iPTH levels remains unclear, although others have proposed that it is related to low dietary phosphorus intakes.
Methods: We conducted a prospective, cross-sectional evaluation of the relationship between age and serum iPTH levels and factors affecting this relationship. All participating subjects were asked to complete a 3-day food diary. The charts were reviewed to obtain routinely measured laboratory values over the preceding 3 months, and serum was collected to measure markers of systemic inflammation.
Results: Ninety-two MHD patients (47 men; age, 51.3+/-14.9 [standard deviation] years; median dialysis vintage, 25.8 months) were studied. Age was inversely correlated with both serum phosphorus and iPTH; these relationships remained significant even when the data were adjusted for diabetic status, dialysis vintage, and dietary nutrient intake. However, there were no associations of age, serum phosphorus, or iPTH with dietary intakes of protein, calories, phosphorus, or calcium either on univariate or multivariate analyses. Markers of systemic inflammation (serum C-reactive protein, and alpha1 acid glycoprotein) did not correlate with age, serum phosphorus, and iPTH or dietary nutrient intake. On the other hand, serum albumin, which may reflect long-term effects of inflammation, did correlate inversely with age and positively with serum phosphorus.
Conclusions: Our cross-sectional study confirms that there are age-related lower levels of both serum phosphorus and iPTH in MHD patients. The mechanisms regarding the inverse relationship between serum phosphorus and age are unclear, but may not be caused by low phosphorus intake or systemic inflammation. In elderly MHD patients, the reduced responsiveness of parathyroid glands may be related to age-dependent accumulation of uremic toxins.