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Angry Endocrinologists Demand Proof Calcium Affects Thyroid Hormone

Doctors demand proof that calcium affects thyroid supplement absorption. If they had searched they would have found a surprising amount of corroboration.

Q. You have written that “Calcium and other minerals found in common antacids can interfere with the absorption of Synthroid.” We believe this to be incorrect. Aluminum hydroxide, Carafate and ferrous sulfate are known to interfere with Synthroid absorption, but we have found no such reference to calcium.

Some of the patients in our endocrinology practice have asked if they should stop their calcium supplements. We can only imagine how many others may have done so without asking their doctors. Please retract this statement or substantiate the interaction.

A. Please refer to the Journal of the American Medical Association (March 11, 1998, p. 750) for a description of three cases in which calcium interfered with levothyroxine absorption.

If you do not accept case reports, how about actual research? Here is a study published in the journal Thyroid (Oct, 2001):

“The administration of calcium and levothyroxine in these subjects was associated with a significant reduction in the peak increment in serum total T4 and free T4 levels, as well as a significant reduction in the overall increment in serum total T4, free T4, and total T3 levels over four time points (120 minutes, 240 minutes, 360 minutes, 1,440 minutes). In summary, this pharmacokinetic study in seven volunteers indicates that calcium carbonate acutely reduces T4 absorption.”

If you think this only applies to calcium carbonate, here is another study in the journal Thyroid (May, 2011) titled “Absorption of levothyroxine when coadministered with various calcium formulations.” Here is what they did:

“The study was conducted in eight healthy, euthyroid [normal] adults. We performed single-dose pharmacokinetic studies in which we measured levothyroxine absorption when given alone or when coadministered with calcium carbonate, calcium citrate, or calcium acetate in doses containing 500 mg elemental calcium. Serum thyroxine was measured at intervals over a 6-hour period after ingestion of the study drugs.


Coadministration of each of the three calcium preparations significantly reduced levothyroxine absorption by about 20%-25% compared with levothyroxine given alone.


Contrary to a prior report, our data suggest that calcium acetate interferes with levothyroxine absorption in a manner similar to that seen with calcium carbonate and calcium citrate. Although the effect of calcium is modest compared with some other medications previously studied, hypothyroid patients should be cautioned to take their levothyroxine well-separated from all of these calcium formulations.”

We have also heard from readers who did not have the expected benefit from their levothyroxine thyroid supplements (Synthroid, Levothroid, Levoxyl) when they took them with vitamins containing calcium or iron or with calcium-fortified orange juice. As a consequence they experienced symptoms of low thyroid activity such as fatigue, constipation, dry skin and hair loss.

Other medicines can also interact with thyroid hormone. We have summarized these together with detailed information about interpreting thyroid tests in our Guide to Thyroid Hormones.

Doctors, we hope this information answers your concerns. We would hope that you will now caution your patients to separate their thyroid dose from calcium, iron, magnesium (Pharmacology & Toxicology, March, 1999) and other minerals by at least two, and preferably, four hours.

Share your own experience with thyroid supplementation below. Were these doctors a little arrogant? We’d like to know what you think.

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About the Author
Joe Graedon is a pharmacologist who has dedicated his career to making drug information understandable to consumers. His best-selling book, The People’s Pharmacy, was published in 1976 and led to a syndicated newspaper column, syndicated public radio show and web site. In 2006, Long Island University awarded him an honorary doctorate as “one of the country's leading drug experts for the consumer.”.
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