FGF23 and vitamin D deficiency: PTH normally results in increased calcium reabsorption and phosphorus excretion by the kidneys; however, this patient also had a vitamin D deficiency, potentially secondary to elevated FGF23 and resulting inhibition of 1-alpha hydroxylase which typically activates inactive 25-OH vitamin D into active 1,25-OH vitamin D. As a result of this diminished vitamin D and decreased bone resorption, the patient was excreting phosphorus more than usual and was unable to maintain normal levels of calcium or phosphorus [15].