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

Two patients with hypothyroidism treated for upper dyspepsia and constipation with aluminum hydroxide and magnesium oxide, respectively, presented a marked increase in the serum concentration of thyroid stimulating hormone and low serum thyroxine on a fixed dosage of levothyroxine. After discontinuation of antacids/laxatives, thyroid stimulating hormone was again reduced indicating interaction between levothyroxine and antacids/laxatives. In vitro studies revealed a dose-related increased adsorption of levothyroxine by addition of a combination of aluminum hydroxide, magnesium hydroxide and magnesium carbonate, while no connection between levothyroxine and the addition of magnesium oxide, alone, was found. This finding has major clinical consequences since 1) many patients are treated with levothyroxine, 2) most patients do not tell physicians that they take antacids/laxatives, and 3) consumption of antacids/laxatives in patients with levothyroxine-treated hypothyroidism may lead to serious undersubstitution with levothyroxine.