Tumor-induced osteomalacia is a rare paraneoplastic syndrome characterized by hypophosphatemia, renal phosphate wasting, suppressed 1,25-dihydroxyvitamin D production, and osteomalacia. It is caused by a usually benign mesenchymal tumor producing fibroblast growth factor 23 (FGF-23). Surgical excision of the tumor is the first choice of treatment because complete resection is curative. Unfortunately, localization often fails due to the small size of these neoplasms. According to the current standards, supportive care with oral phosphate and calcitriol is the only feasible option in such cases.

In this report, we describe the diagnostic value of two-staged venous sampling to localize the FGF-23 secreting tumor in a case where conventional imaging failed. In addition, we examined the effect of dipyridamole on renal phosphate excretion, explored the efficacy of octreotide and calcitonin to suppress the FGF-23 production, and closely evaluated the hormonal changes following successful removal of the tumor. The latter observations indicate that calcitonin may be useful to suppress tumor-FGF-23 production and that FGF-23 may be a clinically relevant inhibitor of parathyroid hormone secretion in man.