Fibroblast growth factor 23 (FGF23) is elevated in patients with PHPT and correlates positively with serum calcium and PTH and negatively with levels of serum phosphorus and 1,25(OH)2 vitamin D.(80) However, in a multiple regression analysis only serum calcium and creatinine clearance were predictors of FGF23, and serum levels of FGF23 did not change after curative surgery, suggesting that this hormone is not likely to play a major role in mediating the hypophosphatemia seen in PHPT.(80) Vitamin D deficiency is also frequently seen in PHPT and can exacerbate its severity.(81, 82). The gene discussed is FGF23; the disease is hypophosphatemia.