Iron, inflammation, and fibroblast growth factor 23: Integrating molecular crosstalk, cardiovascular biology, and therapeutic perspectives

Table 2 Biological information provided by intact and C-terminal fibroblast growth factor 23 measurements

Measurement	What the assay detects	Biological information	Main interpretive limitation
iFGF23	Full-length FGF23; antibodies recognize epitopes on opposite sides of the cleavage site	Circulating biologically active hormone available for receptor activation	May remain stable despite markedly increased production when cleavage rises in parallel[36,37]
cFGF23	Intact hormone plus C-terminal fragments; antibodies recognize epitopes downstream of the cleavage site	Composite signal influenced by both production and proteolytic processing	Does not measure C-terminal fragments selectively and should not be interpreted as intact hormone[36,37]
Paired iFGF23 and cFGF23	Both measurements obtained in the same clinical or experimental setting	Supports interpretation of discordant assay responses; does not directly quantify production, cleavage, or fragments	Assay methods are not standardized or interchangeable. Research interpretation, not a direct cleavage-rate measure[36,37]
C-terminal fragments	Not selectively quantified by current cFGF23 assays	Potential modulation of BMP-hepcidin signaling and iron availability	Biological activity remains supported mainly by experimental evidence. Mouse and cellular evidence[4]

BMP: Bone morphogenetic protein; cFGF23: C-terminal fibroblast growth factor 23 assay result; FGF23: Fibroblast growth factor 23; iFGF23: Intact fibroblast growth factor 23.