fibroblast growth factor 23: a hormone that is central to phosphate homeostasis. It is synthesized by osteoblasts and osteoclasts in response to high phosphate intake, hyperphosphataemia, or an increase in serum *calcitriol concentration. It inhibits phosphate reabsorption by the proximal tubule of the kidney and stimulates 24-hydroxylase, the enzyme that converts calcitriol and its precursor, 25-hydroxy vitamin D, into inactive metabolites. It may also have a negative effect on parathyroid hormone synthesis.
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