NONSPECIFIC INTESTINAL ADSORPTION OF LEVOTHYROXINE BY ALUMINUM HYDROXIDE

PURPOSE: TO evaluate the adverse effect of aluminum hydroxide on levothyroxine pharmacokinetics in hypothyroid patients and study the mechanism of this effect. DESIGN: An in vivo open study supplemented by in vitro experiments. SETTING: A university hospital-based outpatient service. INTERVENTION: Administration for 2 to 4 weeks of an aluminum hydroxide-containing preparation to patients balanced on replacement thyroxine therapy. MEASUREMENTS: Serum thyrotropin (TSH). RESULTS: A significant increase in serum TSH was observed during seven periods of aluminum hydroxide administration (7.19 +/- 1.3 versus 2.62 +/- 0.8 mU/L; P = 0.012). In vitro studies indicated a considerable nonspecific adsorptive capacity of aluminum hydroxide for thyroxine. CONCLUSIONS: The results indicate an adverse effect of aluminum hydroxide on levothyroxine bioavailability through a mechanism involving nonspecific adsorption, or complexing, of levothyroxine to aluminum hydroxide. We recommend close monitoring of serum TSH levels in patients receiving oral thyroid hormone replacement therapy who concurrently take aluminum hydroxide-containing medications. Adjustment of the levothyroxine dose, or cessation of the antacid, may be necessary.