Fig. 1. Simplified schematic presentation of the interactions of parathyroid hormone (PTH), 1,25-dihydroxy-vitamin D (1,25[OH]2D), and fibroblast growth factor-23 (FGF-23) that regulate the calcium (Ca) and phosphate (Pi) homeostasis. The parathyroid glands operate as the command center. PTH induces 1,25(OH)2D and FGF-23 production, increases Ca re-absorption and Pi excretion in the kidney and stimulates their release from bone; 1,25(OH)2D increases Ca and Pi absorption in the intestine, and FGF-23 levels, but have inhibitory effect on PTH synthesis; FGF-23, in the presence of the obligate co-receptor α-klotho, inhibits Pi reabsorption in the kidney, increases production and catabolism of 1,25(OH)2D and production and secretion of PTH. IGF-1, insulin-like growth factor 1.
