Endocrine Abstracts

Case history: A 67-year-old lady was referred to endocrinology with an 8-year history of mild hyperparathyroidism with Parathyroid hormone of 7.4 -15.3 pmol/l (Normal -1.95-8.49). Her corrected calcium ranged from 2.46-2.66 mmol/l (Normal 2.2 -2.62), phosphate 0.63 -1.0 mmol/l (normal 0.8-1.5), normal renal function and normal vitamin D. Her urine calcium to creatinine ratio was 0.7 mmol/mmol (Normal 0-0.4).

Investigations and treatment: She had myalgia due to severe osteoporosis and her DEXA scan showed T score of -2.8(hip), -4.5(spine). She had lost about 1.5 cm from maximal adult height, despite Alendronic acid treatment. She had a negative sestimibi scan and was referred to the tertiary metabolic bone centre to identify the cause for her severe osteoporosis. She was ex-smoker, quit smoking at 34 years of age, attained menarche at 11 years and menopause at the age of 48 by hysterectomy. Tertiary team recommended genetic testing to look for familial hypercalcaemia and hypocalciuria which came back as negative. On review back in endocrinology clinic it was noted she had an intermittent mild hypophosphatemia and on further testing a reduced renal tubular reabsorption of phosphate Tmp/gfr- 40% (0.624) mmol/lGF (0.8-1.35). Her Fibroblast growth factor 23 (FGF23) was elevated at 473iu/ml (0-100). She was referred to the tertiary centre for investigation into tumour induced osteomalacia, she is currently awaiting for a 68- Gallium DOTATE PET scan in February 2023.

Conclusion: The elevated FGF23 and low phosphate levels suggest the possibility of oncogenic osteomalacia. FGF23 is a hormone which regulates serum phosphate levels and excess production of it causes several hypophosphatemic disease. It is also responsible for autosomal dominant hypophosphatemic rickets/osteomalacia. FGF23 was identified as causative humoral factor for tumour induced osteomalacia. It is characterised by low renal tubular reabsorption of phosphate and low serum Dihydroxyvitamin D levels for hypophosphatemia. FGF23 causes low serum phosphate by suppressing the phosphate reabsorption in the renal tubule and in the intestine.