Poster Presentation ESA-SRB-ANZBMS 2024 in conjunction with ENSA

Normalisation of serum phosphate after parathyroidectomy in tertiary hyperparathyroidism complicating X-linked hypophosphataemia   (#399)

Jessie McClelland 1 , Robert Schmidli 2
  1. Department of Endocrinology and Diabetes, Canberra Health Services, Garran, ACT, Australia
  2. Department of Endocrinology and Diabetes, Canberra Health Services, Garran, ACT, Australia

A 66-year-old female was diagnosed at age 3 years with X-linked hypophosphataemia (XLH). This has been complicated by lumbar and cervical spinal stenosis, osteoarthritis and tertiary hyperparathyroidism. Her two children have been diagnosed with XLH, and her daughter has confirmed PHEX c.981C>A gene mutation.

 

Since 2000 she has been managed on low dose phosphate and calcitriol, with phosphate levels ranging between 0.63 and 1.3mmol/L. Parathyroid hormone has been noted to be high since 2007, and hypercalcaemia first developed in 2013. TMP/GFR was 0.5mmol/L and fibroblast growth factor 23 (FGF23) was 951ng/L. Burosumab was started in September 2023 and serum phosphate normalised, however corrected calcium levels rose to a maximum of 3.04mmol/L with PTH 50pmol/L. She subsequently underwent bilateral neck exploration in May 2024 with bilateral superior and partial right inferior parathyroidectomy, and histopathology confirmed parathyroid hyperplasia. Post operatively her serum phosphate levels have normalised without use of phosphate supplementation or burosumab.

 

XLH is due to loss of function mutations in the PHEX gene which leads to increased secretion of FGF-23 (1-3). Higher FGF-23 levels lead to hypophosphataemia through increased renal phosphate wasting and decreased synthesis of 1,25 dihydroxycholecalciferol (1-3).

 

Phosphate supplementation and calcitriol form the cornerstone of conventional treatment (1). However, these medications can further stimulate FGF-23 secretion leading to the complications of nephrocalcinosis and secondary hyperparathyroidism (1-3). Burosumab is a novel humanised monoclonal antibody directed against FGF-23, and has been shown to normalise serum phosphate levels, reduce stiffness and pain, and increase fracture healing (4-6).

 

In adults, secondary hyperparathyroidism affects over 80% and tertiary hyperparathyroidism occurs in 10-30% and can involve multiple glands. (7-9). Hypocalcaemia and hungry bones syndrome may occur postoperatively (9). We report the novel finding of normalisation of phosphate levels in a patient with XLH complicated by tertiary hyperparathyroidism who underwent parathyroidectomy.

 

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