Abstract
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 15years, respectively. After the diagnosis of ADPKD, the 1,25(OH)2D levels increased up to 282pg/mL treated with a pharmacological dose of 1αOHD3 (1αOHD3). At the age of 17years, under a physiological dose, serum 25-hydroxyvitamin D3 (25OHD3) and 1,25(OH)2D levels were 14.0ng/mL and 93pg/mL, respectively. With off-therapy for 3days, the latter decreased to 9pg/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 873pg/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.
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