A 73 year old woman was referred because of hypophosphataemia and raised serum alkaline phosphatase. She was a chronic hepatitis B carrier and regular monitoring had shown a persistently raised alkaline phosphatase (415-495 U/l; normal range 47-124). All other liver enzymes were within the normal range, and ultrasound of the liver was unremarkable.

Further assessment showed low serum phosphate (0.51-0.63 mmol/l; 0.88-1.45), but normal albumin adjusted calcium (2.53-2.63 mmol/l; 2.24-2.63) and creatinine (40 µmol/l; normal 49-82). Her serum parathyroid hormone was extremely high at 689 ng/l (normal 9-52), serum calcidiol (25-hydroxyvitamin D) was low at 23.5 nmol/l (normal >50), and serum calcitriol (1,25-dihydroxycalciferol) was raised at 240.5 pmol/l (60.2-158.6).

She was diagnosed with vitamin D insufficiency with secondary hyperparathyroidism and was prescribed cholecalciferol 800 IU/day. After three months of treatment, serum phosphate remained persistently low at 0.52 mmol/l and she had developed hypercalcaemia (albumin adjusted calcium 2.73 mmol/l). Her 24 hour urine for calcium was normal (7.1 mmol/day; 2.0-7.4) and parathyroid hormone remained raised at 691 ng/l.