Secondary hyperparathyroidism and phosphate binders

[sab3156]

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In firecracker, it states:

"In the case of both chronic renal failure and malabsorption syndrome, the treatment of secondary hyperparathyroidism includes calcitriol (active vitamin D) and phosphate binders (aluminum hydroxide or calcium carbonate). Elevated serum phosphate levels directly depress serum calcium levels, and phosphate binders are used to decrease the prevailing hyperphosphatemia."

Phosphate binders make pharmacologic sense in hyperparathyroidism due to chronic renal failure, due to the fact that there is hyperphosphatemia. However, I thought secondary hyperparathyroidism due to malabsorption syndrome is characterized by hypophosphatemia. Is this correct? If it is correct, then why would you use phosphate binders in the context of hyperparathyroidism due to malabsorption syndrome?

 

[texan2414]

Heyo,

Secondary hyperparathyroidism is a state of hyperparathyroidism, occurring in patients with renal disease. Remember, look at PTH (not Ca2+) to determine if you have a hyper or hypoparathyroidism.

In chronic renal failure, you have a dysfunctional nephron that is 1) not able to excrete phosphate (causing hyperphosphatemia) and 2) not being able to reabsorb calcium at distal tubule. Not only that, you have deficiency in conversion of Vitamin D to its active Calcitriol form (as it is done by 1-a hydroxylase in the nephron which is dysfunctional in renal failure). Taken together, the hyperphosphatemia, low serum Ca and low Vitamin D stimulate regulatory PTH secretion. So secondary hyperparathyroidism, PTH will be high but Ca2+ will be low due to deficient renal absorption of Ca2+.

In this context, phosphate binders are used to decrease the ionized portion of phosphate so would affect the syndrome by helping to somewhat inhibit PTH release.

 

[sab3156]

Yes but secondary hyperparathyroidism can also be caused by malabsorption, in which case there will be hypophosphatemia, not hyperphosphatemia. In this specific type of secondary hyperparathyroidism, why are phosphate binders used? Is it simply to continue shifting the equilibrium (Phosphate::Ca -> Phophate + Ca) to the right? It seems life threatening to take phosphate binders in this case.