What foods should you avoid with levothyroxine?

• Level E Soybean flour, cottonseed meal, walnuts, and dietary fiber may bind and decrease the absorption of levothyroxine sodium from the gastrointestinal tract. Levothyroxine Sodium, FDA-approved label, effective 2026-03-24
• Level E Calcium carbonate supplements can interfere with the absorption of levothyroxine (Synthroid, Levoxyl, and others), a thyroid hormone used to treat hypothyroidism and thyroid cancer [104-106]. The FDA-approved label for this medication instructs patients who are taking calcium carbonate supplements to avoid taking levothyroxine within 4 hours of taking the supplement [107]. NIH Office of Dietary Supplements, Calcium fact sheet for health professionals
• Level E The labels for some of these products [107,108] warn that iron supplements can reduce the absorption of levothyroxine tablets and advise against administering levothyroxine within 4 hours of iron supplements. NIH Office of Dietary Supplements, Iron fact sheet for health professionals

Text in quotation marks is word for word from the official source. Level E means the position of an expert body or a drug label. How we grade evidence.

Evidence behind the advice

• Level B In a randomized crossover trial in 15 healthy adults, taking magnesium aspartate with levothyroxine cut thyroxine absorption by 12 percent (GMR 0.88, 95% CI 0.81 to 0.95). Clin Transl Sci, 2025
• Level D In hypothyroid patients who needed unusually high levothyroxine doses, stopping a fiber supplement lowered the dose they needed, and in lab tests wheat bran bound the drug. J Clin Endocrinol Metab, 1996

The other side and the limits

• Level E The FDA label for levothyroxine says soybean flour may bind the drug and cut its absorption from the gut. Tofu is a soy food but the label does not name it. Levothyroxine sodium tablets, FDA-approved label, section 7.9, effective 2026-03-24
• Level E The FDA-approved levothyroxine label names walnuts among foods that may bind the drug and lower its absorption. Peanuts are not named. Levothyroxine sodium prescribing information, DailyMed, effective 2026-03-24
• Level E The FDA label for levothyroxine says dietary fiber may bind the drug and decrease its absorption from the gut. Corn bran is a fiber source but the label does not name it or give a dosing gap. Levothyroxine sodium tablets, FDA-approved label, section 7.9, DailyMed set f059a576-bb4b-441d-917e-4338bc3df403, effective 2026-03-24

Levels: A is a meta-analysis of trials, B a randomized trial, C an observational study, D lab work or case reports, E an expert position. How we grade evidence.

The label does not ban these foods. It says soybean flour, walnuts, cottonseed meal and dietary fiber may bind the drug and lower absorption. What matters is telling your doctor when your usual eating changes a lot.

Supplements need more care than foods. NIH says levothyroxine labels ask for at least 4 hours between the tablet and calcium carbonate or iron supplements.

An older study below, in hypothyroid patients who needed unusually high levothyroxine doses, found they needed less of the drug after they stopped a fiber supplement. If you plan a big change, such as starting a fiber supplement or eating much more soy flour or walnuts, tell your doctor so your TSH can be checked.

Evidence behind this note

1. Level E The FDA-approved levothyroxine label warns that walnuts, soybean flour, cottonseed meal and dietary fiber may bind the drug and reduce its absorption. Levothyroxine Sodium, FDA-approved label, effective 2026-03-24
2. Level E Calcium carbonate supplements can block levothyroxine absorption, and the drug label tells patients to keep the two at least 4 hours apart. NIH Office of Dietary Supplements, Calcium fact sheet for health professionals
3. Level E NIH notes that taking iron together with levothyroxine can make the thyroid drug less effective, and labels advise a 4-hour gap. NIH Office of Dietary Supplements, Iron fact sheet for health professionals
4. Level D In hypothyroid patients who needed unusually high levothyroxine doses, stopping a fiber supplement lowered the dose they needed, and in lab tests wheat bran bound the drug. J Clin Endocrinol Metab, 1996

Written from the positions and data on this page, reviewed by the Bioma editorial system. How we verify.

• Dietary fiber: what it does and how much you need
• Walnuts: what the research says
• Soy flour: what the research says
• Tofu: what the research says