Fibroblast Growth Factor 23 (FGF23) is a hormone primarily involved in phosphate metabolism and vitamin D regulation. It is secreted mainly by bone cells (osteocytes and osteoblasts) and acts on the kidneys to reduce phosphate reabsorption and suppress vitamin D synthesis, maintaining phosphate homeostasis. Elevated FGF23 levels may indicate conditions such as chronic kidney disease (CKD), hypophosphatemic disorders, or certain tumors. Testing FGF23 levels can be crucial in assessing phosphate imbalance and related metabolic conditions.