A 76 year old man was referred with hypophosphataemia. He had a history of hypertension and gout. He presented with a six month history of generalised bone pain and lower limb weakness. Examination showed weak hip flexion and extension (power 4/5). Complete blood count, serum glucose, and kidney function tests were normal. Serum calcium was 2.33 mmol/L (normal 2.10-2.60), serum phosphate was 0.49 mmol/L (0.8-1.4), and alkaline phosphatase was 204 U/L (47-124). He had no family history of hypophosphataemia or bone disorders. Parathyroid hormone (PTH) was within the normal range (53 ng/L; normal 11-54). Serum calcidiol was low at 38 nmol/L (50-250), and serum calcitriol was also low at 26.8 pmol/L (65.3-171.9). Maximal tubular reabsorption of phosphate was 0.44-0.49 mmol/L (0.9-1.35). A skeletal survey (radiographs including posteroanterior view of the chest; anteroposterior and lateral views of the whole spine, humeruses, and femora; anteroposterior and lateral views of the skull; and anteroposterior view of the pelvis) showed no evidence of fracture or lytic lesion. Technetium-99m-methylene diphosphonate bone scintigraphy showed multiple hot spots over rib cage, involving the costochondral and costovertebral junctions. He was treated with phosphate 500 mg twice daily and calcitriol 0.25 µg daily. ### 1 What is the most likely diagnosis? #### Short answer The patient has tumour induced osteomalacia, with coexisting vitamin D insufficiency. #### Long answer ### 1 Tumour induced osteomalacia Tumour induced osteomalacia, or oncogenic osteomalacia, is a rare acquired metabolic disorder of renal phosphate wasting and hypophosphataemia. Mesenchymal tumours in osteomalacia were first recognised to cause osteomalacia in 1959,1 and more than 120 cases of tumour induced osteomalacia have been reported since then. It is the only paraneoplastic syndrome that affects mineral metabolism and the skeleton. Patients with tumour induced osteomalacia …