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Do Walnuts Really Interfere with Synthroid Effectiveness?

The manufacturer warns patients not to eat walnuts if they take Synthroid. Research studies don't reveal the reason.
Do Walnuts Really Interfere with Synthroid Effectiveness?

Doctors prescribe the thyroid hormone levothyroxine (Levoxyl, Synthroid, Tirosint, Unithroid) for people whose own thyroid glands do not make enough of this hormone. However, too often they don’t provide instructions to help maximize Synthroid effectiveness. For example, many patients go to great lengths to take their pill an hour before breakfast or even (especially) coffee. They don’t know that taking the medication before bedtime is just as effective (Archives of Internal Medicine, Dec. 13, 2010).

Do Foods Alter Synthroid Effectiveness?

Q. How do walnuts affect people who have to take thyroid medication? My pill bottle comes with a warning that walnuts can interfere with the desired medical benefits of levothyroxine. If I eat walnuts several hours after taking my thyroid med, will that solve the problem?

A. We searched the medical literature for an answer to your question. It is surprisingly murky.

The maker of brand name Synthroid (levothyroxine) cautions that

“certain foods and supplements can interfere with Synthroid. Foods containing soy and cottonseed meal can make Synthroid less effective. Walnuts, grapefruit juice, and dietary fiber can also make Synthroid less effective. If you eat any of these on a regular basis, check with your doctor. He or she may need to adjust your dose of Synthroid.”

That seems clear cut, but the research supporting this statement is a bit confusing. We could not confirm that walnuts reduce levothyroxine absorption. If, instead, they interfere with thyroid function, eating them at a different time would not solve the problem.

Grapefruit Juice Has a Minimal Effect:

We were also puzzled by the manufacturer’s reference to grapefruit juice. The only study we could find compared absorption of Synthroid with grapefruit juice or water (British Journal of Clinical Pharmacology, Sep. 2005).

The researchers concluded:

“Grapefruit juice may slightly delay the absorption of levothyroxine, but it seems to have only a minor effect on its bioavailability. Accordingly, the clinical relevance of the grapefruit juice-levothyroxine interaction is likely to be small.”

Other Foods That Interfere With Synthroid Effectiveness:

Some foods do reduce Synthroid effectiveness. A recent review found that absorption is lower when a person takes Synthroid along with coffee, soy products or fiber (Pharmaceuticals, March 2, 2021). Moreover, dietary supplements, particularly calcium and iron, also reduce absorption.

Learn More:

To learn more about thyroid dysfunction and its treatment, including other foods that interfere with levothyroxine, you could consult our eGuide to Thyroid Hormones. You might also want to listen to our interviews with experts on treating thyroid disease. Show 1196: What to Do If Thyroid Treatment Doesn’t Work for You features Dr. Antonio Bianco.  Dr. David Cooper is the expert we interviewed for Show 1162: How too Treat Common Thyroid Problems.

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About the Author
Terry Graedon, PhD, is a medical anthropologist and co-host of The People’s Pharmacy radio show, co-author of The People’s Pharmacy syndicated newspaper columns and numerous books, and co-founder of The People’s Pharmacy website. Terry taught in the Duke University School of Nursing and was an adjunct assistant professor in the Department of Anthropology. She is a Fellow of the Society of Applied Anthropology. Terry is one of the country's leading authorities on the science behind folk remedies. .
  • Bolk N et al, "Effects of evening vs morning levothyroxine intake: a randomized double-blind crossover trial." Archives of Internal Medicine, Dec. 13, 2010. DOI: 10.1001/archinternmed.2010.436
  • Lilja JJ et al, "Effects of grapefruit juice on the absorption of levothyroxine." British Journal of Clinical Pharmacology, Sep. 2005. DOI: 10.1111/j.1365-2125.2005.02433.x
  • Wiesner A et al, "Levothyroxine interactions with food and dietary supplements-A systematic review." Pharmaceuticals, March 2, 2021. DOI: 10.3390/ph14030206
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