Fibroblast Growth Factor 23 (FGF-23)

FGF-23 is a phosphate-lowering hormone. FGF-23 stands for Fibroblast Growth Factor 23. The hormone is produced by bone cells when phosphate metabolism is disturbed or calcitriol and/or parathyroid hormone are elevated in the blood. FGF-23 increases phosphate excretion in the kidneys by reducing the retention of phosphate in the tubular system. Like parathyroid hormone, it also inhibits the phosphate transport systems (NaPi2a & NaPi2c) in the tubular system, which are responsible for the retention of phosphate from the primary urine. As a result, more phosphate is excreted, which lowers the phosphate level in the blood.

FGF-23 also reduces parathyroid hormone and thus the release of phosphate and calcium from the bone. In addition, FGF-23 inhibits the formation of calcitriol, which lowers the blood calcium level. In later stages of chronic kidney disease in humans, an increased blood level of FGF-23 leads to secondary hyperparathyroidism (SHPT) via the reduction of calcitriol and thus to excessive bone resorption by parathyroid hormone with the resulting mineral & bone disorder (MBD). The inhibitory effect of fibroblast growth factor 23 (FGF-23) on the production and release of parathyroid hormone decreases with increasing chronic kidney disease (CKD) stage. Conversely, FGF-23 has a preventative effect on bone mineralisation.

If kidney performance decreases, the FGF-23 blood level rises progressively. In the late stages of CKD, FGF-23 is therefore exponentially higher than at the beginning. High FGF-23 levels are already prognostically unfavourable in early IRIS stages. The blood concentration of FGF-23 is increased when the phosphate balance becomes unbalanced. This may already be the case in the very early stages of CKD – even if the phosphate blood concentration is still within the normal range at this time due to the phosphate-excreting effect of FGF-23. In human medicine, FGF-23 is considered a marker for the progression of chronic kidney disease and is also associated with increased mortality in chronic kidney disease due to its diverse pathological properties.

By measuring FGF-23 again as part of the kidney profile blood test, it is possible to diagnose a derailment of the phosphate metabolism at a very early stage so that phosphate can then be reduced. If the phosphate level in the blood falls, the FGF-23 blood concentration also falls.