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.