October 2023

Levothyroxine Interactions: Common Ingredients to Avoid

Levothyroxine is one of the most prescribed drugs in the US, but many people don’t realize that it can interact with common ingredients in foods and supplements. Make sure you’re counseling patients about how to avoid these interactions.

Taking calcium or iron at the same time as levothyroxine can reduce how much levothyroxine the body absorbs, ultimately reducing its effectiveness. Counsel patients to take iron at least 2 hours apart and calcium at least 4 hours apart from levothyroxine to avoid this. And remember that calcium and iron are both commonly found in multivitamins and other multi-ingredient products – remind patients to check product labels and to separate them from levothyroxine appropriately.

While concerns about grapefruit are fairly well-known among providers, continue to reiterate them to patients, particularly now that grapefruit is being increasingly marketed as a super fruit. Clinical research shows that grapefruit juice can decrease blood levels of levothyroxine by 11%. Tell patients to avoid drinking grapefruit juice 4 hours before or after taking levothyroxine to avoid this interaction.

Coffee might also reduce the effectiveness of levothyroxine. Advise patients to avoid taking levothyroxine and drinking coffee at the same time – drinking coffee one hour after taking levothyroxine doesn’t seem to be a concern.

Finally, watch out for vitamin C – especially as cold and flu season approaches. Two clinical studies show that taking vitamin C and levothyroxine at the same time reduces thyroid stimulating hormone (TSH) levels and increases thyroxine (T4) levels when compared with taking levothyroxine alone. This suggests that vitamin C increases the absorption of levothyroxine, potentially leading to increased effects and side effects.

We’ll keep you up to date as new and notable interactions arise – we’re constantly adding new interactions to our monographs as they are identified. In the meantime, make sure to take advantage of our Interaction Checker.

The information in this brief report is intended for informational purposes only, and is meant to help users better understand health concerns. This information should not be interpreted as specific medical advice. Users should consult with a qualified healthcare provider for specific questions regarding therapies, diagnosis and/or health conditions, prior to making therapeutic decisions. Copyright © 2023 NatMed. Commercial distribution or reproduction prohibited. NatMed is the leading provider of high-quality, evidence-based, clinically-relevant information on natural medicine, dietary supplements, herbs, vitamins, minerals, functional foods, diets, complementary practices, CAM modalities, exercises and medical conditions. Monograph sections include interactions with herbs, drugs, foods and labs, contraindications, depletions, dosing, toxicology, adverse effects, pregnancy and lactation data, synonyms, safety and effectiveness.