FGF23 and secondary hyperparathyroidism: Chronic hypophosphatemia can be caused by several conditions, such as fibroblast growth factor 23 (FGF23)-dependent hypophosphatemia, which can be genetic (caused by mutations of the phosphate regulating gene, PHEX) or acquired, and it is characterized by various symptoms, including reduced absorption of phosphate (due to vitamin D deficiency or resistance) and increased urinary excretion (due to primary or secondary hyperparathyroidism or primary reabsorption deficiency).