Is fibroblast growth factor 23 a harbinger of mortality in CKD? 
Fibroblast growth factor 23 (FGF23) is a novel hormone produced by bone with known functions to regulate urinary phosphate excretion, as well as vitamin D and PTH production.  The discovery of this hormone roughly a decade ago has revolutionized the traditional theories regarding the mechanisms responsible for the mineral metabolism abnormalities that are commonly observed in patients with chronic kidney disease.  Circulating FGF23 levels begin to rise in the early stages of kidney injury and become markedly elevated as kidney disease progresses.  Recent reports have emerged which link these elevations in circulating FGF23 to multiple adverse outcomes.  Most notably, a strong association between increments in FGF23 and cardiovascular pathology has been suggested in patients with both normal and abnormal renal function.  Despite a growing body of evidence to suggest FGF23 as a contributor to morbidity and mortality in CKD, a cause-effect relationship for this association has not been established.  This review highlights our current understanding of the regulation and function of FGF23 and examines the existing literature linking FGF23 with adverse outcomes.