The People's Perspective on Medicine

Are Soy Products a Problem for People on Synthroid?

People on Synthroid should be aware of interactions with coffee, walnuts and grapefruit juice as well as soy milk or soy supplements.

Levothyroxine (Synthroid) is one of the most commonly prescribed medications in the country. Approximately 20 million Americans have a thyroid condition, so perhaps we shouldn’t be surprised that synthetic thyroid hormone is so widely used. There is significant confusion about the best way to take this medicine, however. What should people on Synthroid know about their drug?

Soy Milk Could Interfere with Synthroid Absorption:

Q. I thought soy milk was supposed to be a healthy food. My doctor diagnosed me with a low thyroid condition. I had checked to make sure there weren’t any food interactions with the Synthroid I was prescribed and didn’t see anything worrisome.

Then I started drinking a cup of soy milk per day. Over just a few months, I gained 25 pounds and my TSH reading rocketed up to four times what it had been.

I’ve been unable to take that weight gain off even with increasing dosages of Synthroid. My body temperature is low and I have debilitating fatigue. Are there supplements I should avoid?

A. If you take your Synthroid at the same time as soy milk or coffee, you may not absorb the full dose (Skelin et al, Clinical Therapeutics, Feb. 2017). Digestive disorders such as celiac disease, gastritis and lactose intolerance may also interfere with absorption. Your continuing symptoms suggest that you may not have restored your thyroid hormone to an adequate level. You may need to change your schedule so that you leave at least an hour between swallowing your pill and drinking your beverage.

People on Synthroid Should Check Their Supplements: recently summarized some interactions between Synthroid or levothyroxine and certain supplements. In addition to soy, minerals such as iron or calcium and antacids such as aluminum hydroxide or magnesium hydroxide can interfere with levothyroxine absorption. You should leave at least four hours between swallowing any of these and taking your thyroid hormone. Walnuts and grapefruit juice may also reduce or delay levothyroxine absorption (Synthroid Prescribing Information).

Learn More:

You can learn more about treating your thyroid problem from our Guide to Thyroid Hormones, available at It describes the symptoms of thyroid gland imbalance and how to interpret blood tests for thyroid function. In addition, people on Synthroid can read about foods and medicines that may interfere with the absorption of this important medication.

Changing the Timing:

If taking your Synthroid in the morning is too complicated, you might consider taking it at bedtime. Many people find that easier than trying to take their pill at least an hour before their morning coffee and their supplements (Bolk et al, Archives of Internal Medicine, Dec. 13, 2010).

If that is not appealing, you might ask your doctor about a liquid formulation such as Tirosint. This form of levothyroxine does not interact significantly with coffee (Vita et al, Endocrine, Feb. 2013). It also appears less likely to interact with other drugs (Guglielmi et al, Endocrine, online Sep. 14, 2017).

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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. .
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My TSH levels are stable but as I’ve gotten older I drug holiday one day per week with my docs blessing because I am sensitive to the lower normal TSH levels.
In order to avoid any food interactions with my Levoxyl I now take it when I get up to go to the bathroom during the night. If I don’t get up I either take it with water on arising or with my black, unsweetened coffee. I have never had a problem with an elevated TSH level now since 1989.

Please explain how being lactose intolerant affects the absorption of Synthroid. I have been hypothyroid for 25 years and have thought I had read all the issues with Synthroid, but this is new to me. It is relevant since I have recently become lactose intolerant in my late 40s.

I am not certain, as the reference we found did not specify why this is a problem. I know some doses of Synthroid contain lactose as an excipient (filler), and that may be relevant.

Do the same dietary restrictions apply when taking Armor Thyroid? I know that soy, calcium and iron supplements and walnuts, shouldn’t be consumed for at least four hours after taking Synthroid, but I can’t fine anything definitive about Armor. Thanks.

Victoria, we don’t seem to have studies to tell us if this is a problem with Armour. Possibly not, but we can’t say for sure.

Since most soy is GM, I avoid it as much as possible in all forms.

I take Armour Thyroid. Are the interactions the same? I am vegetarian and eat a LOT of soy

Since hearing the suggestion to take thyroid supplement at bedtime from the People’s Pharmacy, I started taking it in the middle if the night when I regularly wake up to go to the bathroom. It has completely liberated me from always having to think about the timing of taking the pill relative to eating or drinking. I can’t thank you enough for that simple but life changing idea.

I found that drinking soymilk (I really like the flavor!) create hypothyroid symptoms. For over 15 years I have taken the same dose of Armour thyroid for hypothyroidism. As I added more soymilk to my diet (just a cup or two a day) I found hypothyroid symptoms returning (feeling cold a lot and not being able to get warm). I take my thyroid medication several hours away from any food intake. Stopping soy products completely stopped my symptoms.

If Synthroid does not work for you, ask your doctor to switch you to Armour Thyroid, a more complicated, natural version of thyroid hormone. For some people Synthroid does not help with their symptoms.

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