Thyroid hormone (drug interactions)
Thyroid hormone medications, used primarily to treat hypothyroidism, can be significantly affected by various drug interactions and dietary components. Key medications in this category include levothyroxine, liothyronine, and thyroglobulin. It has been observed that certain supplements and foods can interfere with the absorption of these hormones. For instance, calcium and iron should not be taken concurrently with thyroid medications, as they can inhibit absorption; a gap of two to three hours is recommended. Additionally, soy products may disrupt thyroid function, making it crucial for individuals on thyroid medications to monitor their soy intake. On the other hand, L-carnitine supplementation might alleviate some side effects associated with high doses of thyroid hormones. Other potential interactions include biotin, iodine, and vitamin C, which may also influence the effectiveness of thyroid treatments. Furthermore, certain foods—such as those high in gluten, alcohol, or iodine—can hinder the absorption of these medications. Understanding these interactions is essential for optimizing thyroid hormone therapy and ensuring effective treatment.
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, Levoxine, 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.
Kausel, Ana M. "How to Take Your Thyroid Medication." VerywellHealth, 12 May 2022, www.verywellhealth.com/how-to-take-your-thyroid-medication-3231861. Accessed 27 Sept. 2024.
Preston, Claire L. Stockley’s Drug Interactions. 12th ed., London, 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. 989–98. doi.org/10.4158/EP171761.RA. Accessed 17 Nov. 2024.
Shahid, Muhammad A., Muhammad A. Ashraf, and Sandeep Sharma.. "Physiology, Thyroid Hormone." StatPearls, 5 June 2023, www.ncbi.nlm.nih.gov/books/NBK500006. Accessed 27 Sept. 2024.
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.