FGF23 and Hypocalcemia: Hypocalcemia (10, 11), hyperphosphatemia (12), and decreased serum 1,25-dihydroxyvitamin D (1,25(OH)2D) (13, 14) can stimulate the secretion of PTH, while elevated serum calcium (15), 1,25(OH)2D or fibroblast growth factor 23 (FGF23) (16) can inhibit PTH secretion.