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
, 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).
Antacids containing aluminum hydroxide and magnesium hydroxide
(such as Maalox
) should be taken at least four hours before or after taking levothyroxine, as they may delay or prevent the absorption of the drug (Synthroid Prescribing Information 2017
There is some evidence that kelp supplements
(such as tablets and capsules) can increase levels of TSH (thyroid-stimulating hormone).
Although often promoted for thyroid health, iodine
supplements will not help your thyroid work better, and may even cause problems, unless you have an iodine deficiency.
Preliminary evidence suggests ashwagandha
may increase thyroid hormone levels, and so should be used with caution in people taking thyroid hormones, or people with hyper
Be aware that high doses of biotin
(as found in some B-complexes or supplements for hair and nails
) may interfere with certain thyroid laboratory tests.
has 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