SummaryBackground and objectives Fibroblast growth factor 23 (FGF23) regulates phosphorus and vitamin D metabolism. Parathyroid hormone (PTH) infusion for 24 hours stimulated FGF23 secretion in healthy volunteers. The extent to which this was due to a direct stimulatory effect of PTH versus an indirect effect of increasing 1,25-dihydroxyvitamin D [1,25(OH) 2 D] levels was unclear.Design, setting, participants, & measurements Changes in FGF23 in 26 adults undergoing 6-hour (1-34) PTH infusion were examined, focusing particularly on the effects of PTH on FGF23 in the early period of infusion before sustained increases in 1,25(OH) 2 D.Results FGF23 levels declined in parallel with serum phosphate during infusion (P,0.05 for both), with both analytes decreasing within the first hour and reaching their respective nadirs at 6 hours. These changes were observed despite no change in 1,25(OH) 2 D levels during the first hour and a significant increase in 1,25(OH) 2 D from baseline after 6 hours (P,0.001). There were no differences in these responses by race. However, modest racial differences in the phosphaturic response to (1-34) PTH were observed (P=0.04 for interaction), with a higher rate of increase in fractional phosphate excretion in blacks than in whites.Conclusions During short-term (1-34) PTH infusion, FGF23 levels decreased in parallel with serum phosphate levels and despite significant increases in 1,25(OH) 2 D. When coupled with the results of prior longer-term infusion studies, these findings suggest that acute increases in PTH initially act to suppress FGF23 secretion, perhaps to mitigate urinary phosphate losses, before the stimulatory effect of 1,25(OH) 2 D on FGF23 eventually begins to predominate.