Hypophosphatemia related to paraneoplastic Cushing syndrome in prostate cancer: cure after bilateral adrenalectomy

We report the case of a 71-year-old man with progressive metastatic prostate cancer in whom simultaneous occurrence of paraneoplastic Cushing syndrome (CS) and tumor-induced osteomalacia (TIO) initially was suspected. However, the evolution of biochemical markers of phosphate metabolism during disease course and after bilateral adrenalectomy argued against the diagnosis of TIO. Despite the persistence of progressive prostate cancer, CS and hypophosphatemia resolved in parallel after bilateral adrenalectomy. Thus, these data suggest that paraneoplastic CS per se was involved in the pathogenesis of hypophosphatemia. Calcitriol and intact fibroblast growth factor 23 (FGF23) levels were within the reference range at onset, which is inappropriate in the setting of severe hypophosphatemia. All parameters of phosphate metabolism normalized after resolution of hypercortisolism. Based on the known suppressive effect of glucocorticoids (GCs) on bone remodeling and the inverse relationship between bone turnover rate and circulating FGF23 levels, we postulate that GCs interfere indirectly with phosphate homeostasis by inducing inappropriate FGF23 production and release. This mechanism could further aggravate the hypophosphatemia resulting from GC-induced inhibition of intestinal phosphate absorption. Studies directed at the identification of the molecular pathways in bone mediating the interference of GCs with phosphate metabolism are warranted.