Tumour in the dark: a challenging case of osteomalacia.

Tumour-induced osteomalacia (TIO), also known as oncogenic osteomalacia, is a rare paraneoplastic syndrome mediated by the overproduction of phosphaturic hormone fibroblast growth factor 23. TIO is most commonly caused by mesenchymal tumours (PMTs), which are typically small, slow-growing and often undetectable on physical examination and conventional imaging techniques. Patients with TIO typically undergo a protracted period of diagnostic workup and medical treatment due to presentation with nonspecific symptoms and difficulty in localising the culprit tumour. During this period, ongoing surveillance is imperative as medical treatment can limit symptom progression, and tumour identification can provide definitive treatment. We report a case of TIO secondary to a PMT, which, despite biochemical diagnosis, medical treatment and serial imaging, took approximately ten years for tumour localisation.