Intact FGF23 (Fibroblast Growth Factor 23) is a key phosphaturic hormone primarily produced by osteocytes and osteoblasts in bone. It plays a central role in phosphate and vitamin D metabolism by reducing renal phosphate reabsorption and suppressing 1α-hydroxylase activity, thereby lowering levels of active vitamin D (calcitriol). FGF23 circulates in two forms: the full-length biologically active (intact) form, and a cleaved (inactive) C-terminal fragment. Measuring intact FGF23 specifically provides a more accurate reflection of active hormone levels and functional signaling in the body compared to total or C-terminal assays, which detect both active and inactive forms.
In clinical settings, intact FGF23 is used as a biomarker for disorders of phosphate metabolism, including X-linked hypophosphatemia (XLH), tumor-induced osteomalacia (TIO), and certain forms of chronic kidney disease (CKD). Elevated levels are often seen in patients with CKD-MBD (mineral and bone disorder), where early FGF23 rise precedes increases in parathyroid hormone (PTH) and phosphate levels. In rare genetic or oncogenic phosphate-wasting syndromes, intact FGF23 testing aids in diagnosis and monitoring, particularly to guide treatments that target FGF23 pathways, such as burosumab.
In research settings, intact FGF23 is studied extensively for its role in bone–kidney endocrine cross-talk, aging, cardiovascular disease (especially left ventricular hypertrophy in CKD), and systemic inflammation. Researchers use it as a biomarker to investigate early mineral dysregulation, cardiovascular risk in renal patients, and its emerging role in metabolic and inflammatory pathways. Accurate measurement of the intact molecule is critical in these studies to ensure reliable correlations with physiological and pathological outcomes.
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