Inappropriate Secretion of Fibroblast Growth Factor 23 Despite Hypophosphataemia with Changes in Bone Turnover Markers in a Girl with Systemic Lupus Erythematosus: Case Report and Review of the Literature

We report an 11-year-old girl with systemic lupus erythematosus (SLE) who showed hypophosphataemia (1.7 mg/dl, normal range: 3.9-5.8 mg/dl), a decrease in the tubular maximum reabsorption of phosphate/glomerular filtration rate (TmP/GFR) (0.77 mg/dl, normal range: 3.4-5.6 mg/dl), and an elevated serum fibroblast growth factor 23 (FGF23) (circulating phosphate-regulatory hormone) concentration (FGF23: 282 pg/ml, normal range: <52 pg/ml) at the onset. The patient was treated with intravenous pulse methylprednisolone, oral prednisolone, mycophenolate mofetil, hydroxychloroquine, and phosphorus supplement. Serum FGF23 concentrations decreased to near the reference value at 5 months after the onset of SLE, and the TmP/GFR (4.61 mg/dl) simultaneously improved. The urinary deoxypyridinoline (bone resorption marker) concentration on admission (18.9 nmol/mmol creatinine, normal range: 75.4 ± 6.8 nmol/mmol creatinine) was greatly reduced, and the bone-type alkaline phosphatase (bone formation marker) concentration (30.6 µg/l, normal range: 58.6 ± 15.3 µg/l) was also reduced during the increase in FGF23 concentrations before steroid therapy was initiated. The reason for the inappropriate secretion of FGF23, despite hypophosphataemia, remains unknown. The findings in our case suggest that changes in bone turnover markers can occur in patients with SLE and excess inappropriate secretion of FGF23, despite severe and persistent hypophosphataemia.