Effect of phosphate binders upon TSH and L-thyroxine dose in patients on thyroid replacement.

Background: While calcium carbonate is known to interfere with the gastrointestinal absorption of levothyroxine, we hypothesized that other phosphate binders would also bind to levothyroxine and decrease bioavailability of levothyroxine in dialysis patients.

Methods: The records of 1,566 patients on hemodialysis who were being treated by the Hypertension, Nephrology, Dialysis, and Transplantation Center (the regional renal referral center for Eastern Alabama, USA) were evaluated. The type of phosphate binder and amount were then correlated (two-tailed Pearson Correlation) to TSH levels, serum phosphorus and the amount of levothyroxine taken. Friedman Test and Wilcoxon Signed Ranks Test were performed to analyze the significance of difference in thyroxine dosing and TSH levels between the different phosphate binders.

Results: Sixty-seven patients were identified who were taking levothyroxine while taking three different kinds of phosphate binders; namely, calcium carbonate, calcium acetate, and sevelamer HCl. We found that the TSH levels of patients on calcium carbonate (P = 0.002) and sevelamer HCl (P = 0.033) were significantly higher than patients on calcium acetate with the difference increasing with time on each binder. Sevelamer was also found to be associated with significantly higher dosing requirement of thyroid replacement than those on either calcium carbonate or calcium acetate (Z = -3.17, P = 0.001).

Conclusion: Sevelamer (but not calcium acetate) in addition to calcium carbonate appears to interfere with the bioavailability of levothyroxine.