New medications which decrease levothyroxine absorption

Medications may sometimes interfere with the intestinal absorption of levothyroxine, primarily by forming an insoluble complex with the thyroid hormone in the intestinal lumen. The goal of this study was to examine the acute effects of three previously unstudied medications on levothyroxine absorption.

We studied the effects of three medications on thyroxine absorption in seven normal volunteers. On each study day, the subjects ingested 1 mg levothyroxine sodium, either taken separately or co-administered with sevelamer hydrochloride (Renagel, a phosphate-binding medication used in the treatment of hyperphosphatemia), chromium picolinate (an over-the-counter nutritional supplement), or ezetimibe (Zetia, a drug used in the treatment of hypercholesterolemia). Serum thyroxine was measured at intervals over a 6-hour period following drug ingestion.

Sevelamer hydrochloride and chromium picolinate each significantly (p < 0.05) decreased the area under the serum thyroxine concentration curve, while ezetimibe had no effect.

Hypothyroid patients taking sevelamer hydrochloride or chromium picolinate should be advised to separate the time of ingestion of these drugs from their thyroid hormone preparation by several hours.