FGF23 and hyperparathyroidism: It has been demonstrated that both PTH expression and secretion were suppressed by FGF23. Since burosumab inhibits the inhibitory effect of FGF23 on PTH, burosumab may result in increased PTH secretion and development of secondary hyperparathyroidism, as is the case in our patient. Further studies are required to define the mechanism and treatment of burosumab-induced hyperparathyroidism.