Fibroblast growth factor 23 (FGF-23) is a recently discovered growth factor produced by osteocytes and behaving as a hormone in phosphate homeostasis.1 FGF-23 decreases phosphate reabsorption in the kidney by down-regulating the expression of sodium-phosphate co-transporters in the proximal tubule. Moreover, FGF-23 inhibits 1a-hydroxylase expression, leading to decreased hydroxylation of 25- hydroxyvitamin D to 1,25-dihydroxyvitamin D. Increased FGF-23 concentrations, due to genetic mutations, cause excessive renal phosphate excretion and inappropriately low plasma 1,25-dihydroxyvitamin D concentrations in several types of hereditary hypophosphatemic rickets. Tumour-induced osteomalacia is a syndrome consisting of hypophosphataemia, due to decreased renal tubular phosphate reabsorption, inappropriately low 1,25-dihydroxyvitamin D, myopathy and osteomalacia, caused by tumour production of FGF-23.
Patients undergoing haemodialysis often have both increased serum phosphate and FGF-23 concentrations. FGF-23 concentrations in these patients appear to be independently associated with mortality.
Taken from Heijboer et al., Ann Clin Biochem 2009; 46: 338–340
It is recommended that the patient is fasted.
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