Therefore, iron deficiency increases levels of FGF23 measured by C-terminal assays (cFGF23) that measure both full-length FGF23 and its C-terminal cleavage fragments, while the full-length FGF23 levels that are detected exclusively by intact FGF23 assays (iFGF23) remain unchanged [10, 11]. The gene discussed is FGF23; the disease is nutritional disorder.