Initial Posting: 2/7/2014    Last Update: 6/11/2019
Answer: Levothyroxine (Synthroid, Levoxyl), liothyronine (Cytomel), and other thyroid hormone drugs for hypothyroidism (underactive thyroid), can be affected by taking supplements and can affect your ability to absorb certain vitamins and minerals. These interactions are explained in the Hypothyroidism article in the Conditions section of our website, and Thyroid Hormone article, which is part of the extensive Drug Interactions section (where you can look up interaction for other drugs you may be taking). Care must be taken with regard to using these supplements, which include calcium, iron, and soy (including soy protein powders and soy isoflavones -- found in some menopause supplements) (which can affect absorption of thyroid hormone drugs), alpha-lipoic acid (which may lower levels of thyroid hormone T3 and increase levels of TSH) and L-carnitine and acetyl-l-carnitine (which can inhibit the activity of thyroid hormones -- and may help people with hyperthyroidism, or overactive thyroid).
Resveratrolhas been shown to increase TSH levels and cause thyroid enlargement in animals, although it's not known whether it effects thyroid function in people.
Long term exposure to high or even moderate amounts of lithium can affect thyroid function.
Also, be aware that some thyroid-boosting supplements, including herbal supplements, have been found to contain thyroid hormones at significant levels, which are not listed and may be due to drug spiking. See the details in our Warnings section.
Certain foods and drinks may also interfere with absorption or bioavailability of levothyroxine. Soybean flour, cottonseed meal, walnuts, and dietary fiber may bind and decrease the absorption of levothyroxine from the gastrointestinal tract; grapefruit juice may delay the absorption and reduce its bioavailability (Synthroid Prescribing Information 2017). There is also some evidence that coffee may bind to and reduce the absorption of levothyroxine; the evidence suggests it may be best to wait one hour after taking this medication before drinking coffee (Benvenga, Thyroid 2008; American Thyroid Association). [Note: It is not known which compound/s in coffee are responsible for this effect. Therefore, it is possible (but not proven) that decaffeinated coffee may also reduce absorption (Wegrzyn, J Acad Nutr Diet 2016).] Green tea may also interfere with thyroid hormones, so it may be best to consume green tea, as well as green tea supplements, at least one hour apart from thyroid medication.
Cruciferous vegetables such as broccoli, brussels sprouts and cabbage contain compounds that can interfere with thyroid function. However, these compounds appear to be deactivated in vegetables that are cooked (McMillan, Hum Toxicol 1986).