The symptoms of muscle weakness, bone pain and fragility fractures can be an indication of osteomalacia. Phosphate is often not considered within osteologic parameters, decreased levels are therefore easily overseen. The additional test for fibroblast growth factor 23 (FGF23) as indicator for tumor-induced osteomalacia (TIO) is still largely unfamiliar.By emphasizing the role of phosphate and furthermore FGF23 in bone metabolism illustrated by the long-term disease process of our clinical case we would like to introduce these parameters to a broader public.We performed a literature search via PubMed and Google Scholar with the relevant key words and summarized the diagnostic and therapeutic information. The studies evaluated were mainly case reports. We present a case report of a 70-year-old patient with TIO and a myopericytoma and retrospectively analyzed the clinical case. The follow-up was 6 months.Our literature search found one case of TIO and evidence of FGF23 among 124 cases of myopericytomas in total. Over 300 cases of TIO are reported. In our case, we retrospectively found an FGF23-secreting myopericytoma in the phosphaturic mesenchymal tumors (PMT) group to be the cause of pseudarthrosis on the right humerus shaft and increasing disablement in a patient with osteomalacia. After surgical resection the patient was mobile again, and the osteologic parameters, especially phosphate, normalized from 0.21 to 1.52 mmol/l.Low phosphate levels are the decisive indication of TIO in our case. Therefore, we should always think of phosphate level control when dealing with osteomalacia. A hypophosphatemia and hyperphosphaturia should be recognized in time and be diagnostically verified. The additional FGF23 test (c-terminal and intact FGF23) should be considered.
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