Serum calcitriol levels in a patient with X-linked hypophosphatemia complicated by autosomal dominant polycystic kidney disease

Serum calcitriol [1,25(OH)₂D] levels are low normal in the presence of hypophosphatemia in X-linked dominant hypophosphatemic rickets (XLH) due to elevated serum fibroblast growth factor 23 (FGF23) levels. We report a peculiar finding of markedly elevated serum 1,25(OH)₂D levels in a patient with XLH complicated by autosomal dominant polycystic kidney disease (ADPKD) and retinitis pigmentosa (RP). She was diagnosed with XLH, ADPKD and RP at the age of 5, 13 and 15 years, respectively. After the diagnosis of ADPKD, the 1,25(OH)₂D levels increased up to 282 pg/mL treated with a pharmacological dose of 1αOHD₃ (1αOHD₃). At the age of 17 years, under a physiological dose, serum 25-hydroxyvitamin D₃ (25OHD₃) and 1,25(OH)₂D levels were 14.0 ng/mL and 93 pg/mL, respectively. With off-therapy for 3 days, the latter decreased to 9 pg/mL with an increased ratio of 24,25-dihydroxyvitamin D₃ (24,25(OH)₂D₃)/25OHD₃. Serum FGF23 and circulating soluble α-klotho (sKlotho) levels were 552 and 873 pg/mL, respectively, with the latter being almost half of the normal control. Since the coexistence of elevated serum FGF23 and 1,25(OH)₂D levels contradicted the conventional FGF23 actions, we speculated on the possible association of decreased sKlotho with the peculiar vitamin D metabolism, although mechanisms in detail remained unknown.