Fibroblast growth factor-23 and renin–angiotensin system levels in vitamin-D-dependent rickets type I

Background: As 1,25(OH)₂D₃ vitamin D3 induces fibroblast growth factor-23 (FGF-23) production and suppresses the renin–angiotensin–aldosterone system (RAAS), its absence in vitamin-D-dependent rickets type I (VDDR-I) may have adverse health consequences. Case description: An infant presented at age 8 months with hypocalcemia and rickets and very low 1,25(OH)₂D₃ levels. Genetic analysis confirmed VDRR-I, and calcitriol therapy was initiated. During periods of nonadherence to therapy, chemical measurements revealed detectable FGF-23 levels, with undetectable 1,25(OH)₂D₃, hypophosphatemia, low tubular reabsorption of phosphate, hypocalcemia, and very elevated parathyroid hormone (PTH) levels. These changes, in addition to elevated RAAS levels, normalized during calcitriol therapy despite elevated FGF-23 levels. At age 12 years, all rachitic manifestations were absent, and bone mineral density (BMD) and the echocardiogram were normal. Conclusions: Whereas 1,25(OH)₂D₃ is not indispensable for FGF-23 production, PTH in the absence of vitamin D may maintain FGF-23 secretion despite hypocalcemia. Normalization of urinary phosphate losses despite elevated FGF-23 during calcitriol-mediated suppression of secondary hyperparathyroidism points to a cardinal role of PTH as a cause of the phosphaturia in VDRR-I. Normalization of RAAS by calcitriol may conceivably prevent adverse cardiovascular outcomes.