Endocrinology

Answer 5 Potassium chloride. Levothyroxine is used in patients who are thyroid hormone-deficient. Approximately 80% of the total levothyroxine dose is absorbed by the jejunum and ileum within the first 3 hours of administration. Because there is a narrow therapeutic window for levothyroxine dosing, patients require frequent TSH monitoring to maintain a biochemical euthyroid state. Those who absorb less levothyroxine may have incompletely treated hypothyroidism. Drugs that have been shown to decrease the absorption of levothyroxine include antacids that contain aluminum hydroxide,8 bile acid sequestrants,8 calcium carbonate,9 and ferrous sulfate.8 Potassium chloride has not been shown to interfere with the absorption of levothyroxine. REFERENCE 8.  Surks MI, Sievert R. Drugs and thyroid function. N Engl J Med 1995;333:1688-94. 9. Singh N, Singh PN, Hershman JM. Effect of calcium carbonate on the absorption of levothyroxine. JAMA 2000; 283:2822-5. Click here to return to the questions

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