Endocrine Abstracts

A 77 year old female was referred to endocrinology with an incidental finding of hypophosphataemia (0.26 mmol/l) on routine bloods. She described a slight unsteadiness on her feet, but denied bone pain or overt muscle weakness. Past medical history included Type 2 Diabetes Mellitus, a left humeral fragility fracture and the subsequent diagnosis of osteoporosis 2 years previously. At presentation the corrected calcium was slightly elevated (2.64 mmol/l), which normalised when repeated, with suppression of parathyroid hormone (0.8 pmol/l) and adequate 25-hydroxyvitamin D concentrations (71 nmol/l). Renal function was normal and no paraproteins were detected. Phosphate levels were suboptimal for approximately 3 years, however, had been normal prior to this. FGF23 was found to be significantly elevated (186 RU/ml; normal range <100). An octeotide scan was undertaken demonstrating the presence of a moderately octreotide avid heterogenous soft tissue mass lesion within the right thigh. A successive MRI confirmed the presence of a 7.2×3.8×6 cm well-defined infiltrative enhancing mesenchymal tumour of the right adductor musculature. Tumour induced osteomalacia is a rare paraneoplastic disorder characterised by hypophosphataemia due to decreased renal tubular reabsorption of phosphate as a result of tumour FGF23 overproduction. The majority of tumours responsible for this condition are phosphaturic mesenchymal tumours of the mixed connective tissue variant. These tumours are often small, slow growing, occur in diverse locations and are largely benign, however, can metastasise. Other tumours associated include osteosarcomas and advanced metastatic cancers of the colon and prostate. Chronic hypophosphataemia impairs bone mineralisation and can result in significant proximal myopathy, however, patients are often asymptomatic and phosphate depletion is identified incidentally. Consequently, FGF23 is a useful biomarker in the diagnosis of tumour induced osteomalacia, which if resected can be curative.