Clinical_Rounds_in_Endocrinology_Volume_II_-_Pediatric_Endocrinology

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9. What are the regulators of phosphorus homeostasis?

The level of phosphate in the intracellular compartment is one to two times

higher than in the extracellular fluid. The regulators of phosphate homeostasis

are 1,25(OH) 2 D, PTH, and FGF-23. 1,25(OH) 2 D increases the reabsorption of

phosphate from the gut. PTH and FGF-23 promote the excretion of phosphate

by decreasing the renal tubular reabsorption of phosphate. The effect of PTH is

immediate, whereas that of FGF-23 takes several hours. However, PTH and

FGF-23 have opposing effects on renal 1α-hydroxylase activity; PTH stimu-

lates it, whereas FGF-23 inhibits it. PTH promotes bone remodeling and hence

maintains the phosphate exchange between bone and extracellular fluid. The

role of other phosphatonins apart from FGF-23 in phosphate homeostasis is not

precisely defined.

10. How is serum phosphorus maintained in the normal range?

Approximately 90 % of ingested phosphorus is absorbed from the upper intes-

tine (duodenum and jejunum). Out of this, 60–70 % is passively absorbed and

PCT

70%

20%

6%

2%

2%

TALH

CCD

MCD

DCT

Calcium Homeostasis

PCT - Proximal convoluted tubule

DCT - Distal convoluted tubule

TALH - Thick ascending limb of

loop of Henle

CCD - Cortical collecting duct

MCD - Medullary collecting duct

Fig. 5.5 Sites of calcium reabsorption from the kidney

5 Rickets–Osteomalacia