Interpretation of Plasma PTH Concentrations According to 25OHD Status, Gender, Age, Weight Status, and Calcium Intake: Importance of the Reference Values

Context: Reference values for plasma PTH assessment were generally established on small samples of apparently healthy subjects, without considering their 25-hydroxyvitamin D (25OHD) status or other potential modifiers of PTH concentration. Objective: Our objective was to assess ranges of plasma PTH concentration in a large sample of adults, stratifying by 25OHD status, age, gender, weight status, and calcium intake. Design, Setting, and Participants: This cross-sectional survey is based on 1824 middle-aged Caucasian adults from the Supplementation en Vitamines et Mineraux Antioxydants study (1994). Main Outcome Measures: Plasma PTH and 25OHD concentrations were measured by an electro-chemoluminescent immunoassay. Extreme percentiles of plasma PTH concentrations were assessed specifically in subjects who had plasmatic values of 25OHD of 20 ng/mL or greater and 30 ng/mL or greater. Results: Among subjects with 25OHD status of 20ng/mL or greater, the 97.5th percentile of plasma PTH concentration was 45.5 ng/L. By using this value as a reference, 5% of the subjects with plasma 25OHD less than 20 nmol/L had a high plasma PTH level, reflecting secondary hyperparathyroidism. Among vitamin D-replete subjects (25OHD status of 20 ng/mL or greater), the 97.5th percentile of plasma PTH was higher in overweight/obese subjects (51.9 vs 43.5 ng/L among normal weight subjects). Conclusions: The reference value for plasma PTH defined in this vitamin D-replete population was far below the value currently provided by the manufacturer (65 ng/L) and varied according to overweight status. These results may contribute to improve the diagnosis of primary and secondary hyperparathyroidism and subsequent therapeutic indication.