Figure 4.

Vitamin D endocrine system, phosphate homeostasis, and FGF23. Increased serum phosphate or decreased serum calcium levels (not shown) induce PTH secretion by the parathyroid gland, which stimulates renal 1,25-(OH)₂D synthesis. Increased 1,25-(OH)₂D levels induce FGF23 production by osteoblasts and osteocytes. Both increased FGF23 and PTH levels reduce Na/Pi-2a and Na/Pi-2b expression in kidney and intestine, respectively, resulting in decreased phosphate (re)absorption and lower serum phosphate levels. FGF23 also down-regulates renal 1α-hydroxylase and decreases PTH secretion, creating a multiloop feedback system.