Journal or Book Title
Journal of Nutrition
The concentration or threshold of 25-hydroxyvitamin D [25(OH)D] needed to maximally suppress intact serum parathyroid hormone (iPTH) has been suggested as a measure of optimal vitamin D status. Depending upon the definition of maximal suppression of iPTH and the 2-phase regression approach used, 2 distinct clusters for a single 25(OH)D threshold have been reported: 16-20 ng/mL (40-50 nmol/L) and 30-32 ng/mL (75-80 nmol/L). To rationalize the apparently disparate published results, we compared thresholds from several regression models including a 3-phase one to estimate simultaneously 2 thresholds before and after adjusting for possible confounding for age, BMI, glomerular filtration rate, dietary calcium, and season (April-September vs. October-March) within a single data set, i.e. data from the Tufts University Sites Testing Osteoporosis Prevention/Intervention Treatment study, consisting of 181 men and 206 women (total n = 387) ages 65-87 y. Plasma 25(OH)D and serum iPTH concentrations were (mean Â± SD) 22.1 Â± 7.44 ng/mL (55.25 Â± 18.6 nmol/L) and 36.6 Â± 16.03 pg/mL (3.88 Â± 1.7 pmol/L), respectively. The 3-phase model identified 2 thresholds of 12 ng/mL (30 nmol/L) and 28 ng/mL (70 nmol/L); similar results were found from the 2-phase models evaluated, i.e. 13-20 and 27-30 ng/mL (32.5-50 and 67.5-75 nmol/L) and with previous results. Adjusting for confounding did not change the results substantially. Accordingly, the 3-phase model appears to be superior to the 2-phase approach, because it simultaneously estimates the 2 threshold clusters found from the 2-phase approaches along with estimating confidence limits. If replicated, it may be of both clinical and public health importance.
Works produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted.
Durazo-Arvizu, Ramon A.; Dawson-Hughes, Bess; Sempos, Christopher T.; Yetley, Elizabeth A.; Looker, Anne C.; Cao, Guichan; Harris, Susan S.; Burt, Vicki L.; Carriquiry, Alicia L.; and Picciano, Mary Frances, "Three-Phase Model Harmonizes Estimates of the Maximal Suppression of Parathyroid Hormone by 25-Hydroxyvitamin D in Persons 65 Years of Age and Older" (2010). Statistics Publications. 37.