FGF-23 (fibroblast growth factor 23), secreted by osteocytes, regulates phosphate homeostasis. In X-linked hypophosphatemia (XLH), loss-of-function mutations in PHEX lead to excess FGF-23. The downstream molecular pathway explains hypophosphatemia as follows:

• A Excess FGF-23 stimulates PTH secretion from parathyroid → PTH downregulates NaPi cotransporters → phosphaturia and hypercalcemia
• B Excess FGF-23 acts on FGFR1/Klotho complex in renal proximal tubule → downregulates NaPi-IIa and NaPi-IIc cotransporters → phosphaturia; simultaneously inhibits CYP27B1 (1α-hydroxylase) → reduced calcitriol → further phosphate malabsorption ✓
• C Excess FGF-23 inhibits osteocalcin from osteoblasts → impaired bone matrix mineralization → release of phosphate into serum paradoxically causing hyperphosphatemia initially
• D FGF-23 binds FGF receptor 4 on intestinal enterocytes → inhibits TRPV6 calcium channel → hypocalcemia → secondary PTH rise → phosphaturia

Explanation

FGF-23 requires its co-receptor α-Klotho to bind FGFR1 on renal proximal tubular cells. This signaling complex activates intracellular kinase cascades that internalize and degrade the apical sodium-phosphate cotransporters NaPi-IIa (SLC34A1) and NaPi-IIc (SLC34A3), causing phosphaturia. Additionally, FGF-23 suppresses CYP27B1 (renal 1α-hydroxylase) and induces CYP24A1 (24-hydroxylase), both reducing calcitriol synthesis and accelerating its degradation. Low calcitriol impairs intestinal phosphate absorption, worsening hypophosphatemia. In XLH, this combination of renal wasting and reduced intestinal absorption causes rickets/osteomalacia despite normal or high calcitriol precursors (25-OH vitamin D).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.