Relation Between Circulating Levels of 25(OH) Vitamin D and Parathyroid Hormone in Chronic Kidney Disease: Quest for a Threshold

Context: Vitamin D deficiency is common in patients with chronic kidney disease (CKD). Current guidelines recommend treatment strategies in these patients similar to those for the general population, but the vitamin D nutritional status sufficient to prevent PTH levels from increasing in CKD is unknown. Objective, Main Outcome Measure: Our aim was to study the relation between circulating PTH and 25(OH) D levels and to search for a 25(OH) D threshold associated with a significant PTH increase. Design, Setting, and Patients: In the hospital-referred NephroTest cohort study, we measured 25(OH) D, PTH, and glomerular filtration rate(mGFR) by Cr-51-EDTA renal clearance in 929 adult patients with nondialysis CKD stages 1 to 5 and no vitamin D supplementation. Patients' mean age was 60.1 +/- 14.7 years; 71% were men, and 9% were black. Their median mGFR was 37.8 mL/min/1.73 m(2). Results: We found a 25(OH) D threshold of 8 ng/mL with an upper limit of 20 ng/mL(95% confidence interval) by linear piecewise regression modeling of log-PTH for 25(OH) D adjusted for mGFR, age, race, and ionized calcium level. The smoothed curve confirmed that PTH concentration rose steeply when circulating 25(OH) D levels fell to less than 20 ng/mL. Conclusions: Spontaneous 25(OH) D levels greater than 20 ng/mL seem sufficient to control serum PTH in CKD patients. This result reinforces guidelines to supplement vitamin D only if less than 30 ng/mL.