A rare, endocrine active tumor that causes severe renal phosphate wasting, which in turn can lead to critical osteomalacia. Phosphaturic mesenchymal tumors (PMTs) are typically small and mostly benign tumors producing fibroblast growth factor 23 (FGF-23). FGF-23 lowers the expression of sodium/phosphate cotransporters, which are the primary transport proteins responsible for phosphate reabsorption in the kidneys. The paraneoplastic overproduction of FGF-23 lowers reabsorption of phosphate and causes severe paraneoplastic renal phosphate wasting and hypophosphatemia. FGF-23 also affects vitamin D levels by lowering 25-hydroxyvitamin D 1-alpha-hydroxylase in the proximal renal tubules and by increasing the expression of vitamin D 24-hydroxylase, a mitochondrial enzyme responsible for inactivating vitamin D metabolites. [from HPO]

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