Selected Topics From the 73rd Annual Meeting of the American Thyroid Association

Kenneth D. Burman, MD

Disclosures

June 21, 2002

In This Article

Effect of Calcium Carbonate on Levothyroxine Absorption

Exogenous levothyroxine therapy is commonly used for replacement and suppressive purposes, especially in the treatment of hypothyroid and thyroid cancer patients, respectively. It is important that the medication be taken compliantly, usually daily. However, several medications are known to affect the absorption of levothyroxinewhen given concurrently, and may therefore alter thyroid stimulating hormone (TSH) and free thyroxine (T4) and/or total thyroxine (TT4) serum concentrations. Ferrous sulfate, sucralfate, bile acid sequestrants, and aluminum hydroxide-containing antacids are several compounds known to alter thyroid hormone absorption.

In the present study,[1] the authors determined the acute effects of calcium administration on levothyroxine absorption. They assessed serum total T4 and triiodothyronine (T3), free T4, and TSH following the administration of 1000 mcg levothyroxine on 2 different occasions separated by a 4-week interval. At the first study period, only levothyroxine was given, and at the second study period levothyroxine was coadministered with 2 g calcium carbonate. Following the administration of 1000 mcg levothyroxine alone, the maximum calculated average T4 absorption was 837 mcg (83.7% of administered dose) at 2 hours. When levothyroxine was given in combination with calcium, the maximum average T4 absorption fell to 579 mcg (57.9% of the dose ingested) at 240 minutes. These differences were statistically significant, as were the observed decrements in free T4 and TT3.

In summary, calcium carbonate acutely reduces T4 absorption and resultant serum thyroid hormone levels. These authors have previously noted[2] that the oral ingestion of calcium carbonate in patients receiving long-term levothyroxine therapy significantly reduced levothyroxine absorption and increased TSH concentrations.

This information is sufficient to recommend that patients not ingest oral calcium carbonate or, in fact, any of the medications mentioned above, concurrently with levothyroxine. However, we do not know the precise interval that should separate the administration of these 2 medications. Clinically, we recommend an interval of several hours to ensure that goal thyroid hormone levels are achieved.

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