Thyroid hormone (drug interactions)

DEFINITION: Supplements used to treat hypothyroidism, a condition caused by deficient secretion of thyroid hormone by the thyroid gland.

• INTERACTIONS: Calcium, carnitine, iron, soy
• DRUGS IN THIS FAMILY: Dextrothyroxine (Choloxin), levothyroxine (Levothroid, Levoxyl, Synthroid, Tirosint, Unithroid), liothyronine (Cytomel, Triostat), liotrix (Euthroid, Thyrolar), thyroglobulin (Proloid), thyroid (Armour Thyroid)

Calcium

Effect: Take at a Different Time of Day

Two case reports suggest that calcium carbonate interferes with the body’s absorption of thyroid hormone when both are taken at the same time.

Several studies have investigated this interaction. In one small three-month study, twenty individuals with hypothyroidism stabilized on long-term levothyroxine therapy were given calcium carbonate, 1,200 milligrams (mg) daily. While taking the supplement, participants' thyroid hormone levels in the blood declined. However, after stopping supplementation, thyroid levels climbed back up, slightly surpassing pre-study levels. These results imply that calcium reduces the absorption of thyroid hormones and that they should not be taken together.

The thyroid is an important gland that produces thyroid hormone, the proper level of which is crucial to health. The inset shows the location of the thyroid gland.

Iron

Effect: Take at a Different Time of Day

Iron salts used to treat iron deficiencies, like ferrous fumarate, ferrous gluconate, ferrous sulfate, or iron polysaccharide, may impair the body's ability to absorb the thyroid hormone levothyroxine. To ensure these substances do not interact, they should be taken at least two to three hours apart. Foods containing iron, like broccoli, beans, and meats, also have high iron content and should be avoided for two to three hours after taking thyroid hormone medications.

Soy

Effect: Possible Harmful Interaction

Ingesting soy in products like tofu, soy milk, or soy-based infant formula has been shown to interfere with thyroid function and thyroid medications. Individuals taking thyroid medications should be aware of their soy intake and avoid significant increases or decreases in the amounts. Thyroid hormone medications may need to be adjusted for such significant dietary changes.

Carnitine

Effect: Supplementation Possibly Helpful

Some research suggests that L-carnitine supplementation may help reduce the side effects of thyroid hormone medications. If a patient's thyroid is enlarged, high doses of thyroid medication may be necessary to treat the condition. High doses often cause bone loss, heart palpitations, headaches, and symptoms similar to the flu.

Other Interactions

Biotin, iodine, kelp, and vitamin C supplements can interfere with thyroid medications and should be taken with caution. Because of the acidity in coffee and tea, waiting an hour after taking thyroid medication to drink these beverages is advisable. Some foods can impact the absorption of thyroid medications, like soy protein powder, walnuts, grapefruit, orange juice, caffeine, and some fiber supplements. Soy protein and cottonseed meal decrease the effect of the medication Synthroid.

Foods rich in iodine, like seaweed, shrimp, and milk, also reduce the effect of thyroid hormone medications. Foods with high levels of gluten or drinking alcohol can make it more difficult for the body to absorb these medications.

Bibliography

Benvenga, S., et al. “Usefulness of L-carnitine, a Naturally Occurring Peripheral Antagonist of Thyroid Hormone Action, in Iatrogenic Hyperthyroidism.” Journal of Clinical Endocrinology and Metabolism, vol. 86, 2001, pp. 3579-94.

Butner, L. E., et al. “Calcium Carbonate-Induced Hypothyroidism.” Annals of Internal Medicine, vol. 132, 2000, p. 595.

Liu, Hanqing, et al. "Medications and Food Interfering with the Bioavailability of Levothyroxine: A Systematic Review." Therapeutics and Clinical Risk Management, vol. 19, 2023, p. 503-23, doi.org/10.2147/TCRM.S414460. Accessed 2 Jan. 2026.

Preston, Claire L. Stockley’s Drug Interactions. 12th ed., Pharmaceutical Press, 2021.

Samarasinghe, Shanika, et al. “Biotin Interference with Routine Clinical Immunoassays: Understand the Causes and Mitigate the Risks.” Endocrine Practice: Official Journal of the American College of Endocrinology and the American Association of Clinical Endocrinologists, vol. 23, no. 8, 2017, pp. 989–998. Endocrine Practice, doi:10.4158/EP171761.RA. Accessed 2 Jan. 2026.

Shahid, Muhammad A., et al. "Physiology, Thyroid Hormone - StatPearls." National Center for Biotechnology Information, 5 June 2023, www.ncbi.nlm.nih.gov/books/NBK500006. Accessed 2 Jan. 2026.

Shomon, Mary. "How to Take Your Thyroid Medication Including Levothyroxine." Verywell Health, 3 Dec. 2025, www.verywellhealth.com/how-to-take-your-thyroid-medication-3231861. Accessed 2 Jan. 2026.

Singh, N., et al. “Effect of Calcium Carbonate on the Absorption of Levothyroxine.” Journal of the American Medical Association, vol. 283, 2000, pp. 2822-25.