Intestinal adsorption of levothyroxine by antacids and laxatives: Case stories and in vitro experiments

Two patients with hypothyroidism treated for upper dyspepsia and constipati on with aluminum hydroxide and magnesium oxide, respectively presented a ma rked increase in the serum concentration of thyroid stimulating hormone and low serum thyroxine on a fixed dosage of levothyroxine. After discontinuat ion of antacids/laxatives, thyroid stimulating hormone was again reduced in dicating interaction between levothyroxine and antacids/laxatives. In vitro studies revealed a dose-related increased adsorption of levothyroxine by a ddition of a combination of aluminum hydroxide, magnesium hydroxide and mag nesium carbonate, while no connection between levothyroxine and the additio n of magnesium oxide, alone, was found. This finding has major clinical con sequences since 1) many patients are treated with levothyroxine, 2) most pa tients do not tell physicians that they take antacids/laxatives, and 3) con sumption of antacids/laxatives in patients with levothyroxine-treated hypot hyroidism may lead to serious undersubstitution with levothyroxine.