Objective: To determine whether the reference range for parathyroid hormone (PTH) should be lowered (from 65 pg/mL to a proposed value of 46 pg/mL) with use of the Allegro radioimmunometric assay.
Methods: We examined the reference range for PTH, adjusted for serum 25-hydroxyvitamin D (25-OHD), in 503 healthy African American and white women, who were 20 to 80 years old. We also analyzed other factors that are thought to influence PTH levels.
Results: Univariate predictors of PTH were identified, and a multivariate model was developed with use of the variables and PTH. Serum PTH was significantly higher in black study subjects than in white study subjects (P<0.02). Increasing PTH was also significantly correlated with increasing body mass index, age, and serum creatinine and with decreasing dietary calcium intake and serum 25-OHD levels. A stepwise multiple linear regression analysis yielded the following predictors of PTH: body mass index (R2=9.4%), age (R2=1.0%), and serum 25-OHD (R2=0.8%). In our study population, many PTH values were above the proposed new upper limit of 46 pg/mL.
Conclusion: The upper limit of the reference range for serum PTH should not be changed. Factors to be considered in analysis of serum PTH values in the upper reference range in patients with normocalcemia include obesity, race, 25-OHD levels, advanced age, serum creatinine, and dietary calcium intake.