NONSPECIFIC INTESTINAL ADSORPTION OF LEVOTHYROXINE BY ALUMINUM HYDROXIDE

PURPOSE: TO evaluate the adverse effect of aluminum hydroxide on levot hyroxine pharmacokinetics in hypothyroid patients and study the mechan ism of this effect. DESIGN: An in vivo open study supplemented by in v itro experiments. SETTING: A university hospital-based outpatient serv ice. INTERVENTION: Administration for 2 to 4 weeks of an aluminum hydr oxide-containing preparation to patients balanced on replacement thyro xine therapy. MEASUREMENTS: Serum thyrotropin (TSH). RESULTS: A signif icant increase in serum TSH was observed during seven periods of alumi num hydroxide administration (7.19 +/- 1.3 versus 2.62 +/- 0.8 mU/L; P = 0.012). In vitro studies indicated a considerable nonspecific adsor ptive capacity of aluminum hydroxide for thyroxine. CONCLUSIONS: The r esults indicate an adverse effect of aluminum hydroxide on levothyroxi ne bioavailability through a mechanism involving nonspecific adsorptio n, or complexing, of levothyroxine to aluminum hydroxide. We recommend close monitoring of serum TSH levels in patients receiving oral thyro id hormone replacement therapy who concurrently take aluminum hydroxid e-containing medications. Adjustment of the levothyroxine dose, or ces sation of the antacid, may be necessary.