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Absorption of thyroxine is reduced by phosphate-binding drugs in patients with severe kidney disease
The background of the study. Patients with severe kidney disease have high serum phosphate levels, which lower serum calcium levels and lead to deposition of calcium and phosphate in tissues. They are, therefore, often treated with calcium acetate, calcium carbonate, or sevelamer (Renagel), which is calcium-free, to bind phosphate in the intestine and therefore reduce its absorption. In this study, the effect of these compounds on serum thyrotropin (TSH) concentrations was determined in patients with severe kidney disease treated by hemodialysis who also had hypothyroidism and were taking thyroxine (T4).
How the study was done. The records of 1566 hemodialysis patients were searched to identify patients with hypothyroidism who had been treated with T4 and either calcium acetate, calcium carbonate, or sevelamer. The T4 was taken with breakfast and the phosphate binders were taken at breakfast, lunch, and dinner. Serum TSH was measured periodically for two years after the phosphate binder was started.
The results of the study. Sixty-seven hemodialysis patients had hypothyroidism and were taking T4. Thirty-five (52 percent) were treated with calcium acetate, 19 (28 percent) with calcium carbonate, and 13 (20%) with sevelamer. During the two-year study period, the average T4 dose in the calcium-acetate group was 95 μg daily, and their average serum TSH concentration was 3.9 mU/L. The T4 dose in the calcium-carbonate group was 97 μg daily, and their average serum TSH concentration was 23.8 mU/L. The T4 dose in the sevelamer group was higher (137 μg daily), but despite the higher dose their mean overall serum TSH concentration was 20.3 mU/L, indicating inadequate T4 treatment.
The conclusions of the study. In patients with severe kidney disease and hypothyroidism, calcium carbonate and sevelamer decrease the intestinal absorption of T4, thereby increasing their requirement for T4.
The original article. Diskin CJ, Stokes TJ, Dansby LM, Radcliff L, Carter TB. Effect of phosphate binders upon TSH and L-thyroxine dose in patients on thyroid replacement. Int Urol Nephrol 2007;39:599-602.
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| Thyroid Digest Index | | | November 2007 Thyroid Digest |
