Doctors prescribe levothyroxine for hypothyroidism, a medical condition caused by an underactive thyroid. Taking levothyroxine with other drugs or certain foods might have negative consequences. Learning about these interactions can help you safely use this medication to its full effectiveness.
Read more: Diet Plans for People Without a Thyroid
Levothyroxine: Drug Interactions
A number of substances — including other medications, foods and beverages — may interact with levothyroxine and decrease its absorption and effectiveness. Many drugs affect the way your body absorbs levothyroxine, so you should tell your health care provider about everything you take, including nutritional supplements.
The authors of a January 2015 report in Clinical Endocrinology listed several known drugs that can interfere with levothyroxine's absorption. This list features a range of substances including proton pump inhibitors used for acid reflux, estrogen replacement medications and statins used to lower cholesterol levels. In addition, certain mineral supplements, including calcium carbonate, calcium citrate and iron sulfate, will decrease the effects of levothyroxine.
Several medications used for seizure disorders may decrease levothyroxine's effectiveness. Anti-epileptic drugs like phenobarbital and carbamazepine may increase the metabolism of levothyroxine, according to a February 2015 review in Lancet Diabetes and Endocrinology. This means that people with epilepsy and hypothyroidism may need larger doses of levothyroxine.
Antibiotics can also cause drug interactions with varying results. A study published in the November 2013 issue of Thyroid shows this paradox. These researchers gave two antibiotics — ciprofloxacin and rifampin — to eight subjects also given levothyroxine.
Ciprofloxacin decreased thyroid hormone levels by 39 percent, and rifampin increased them by 25 percent. If you need to use levothyroxine and antibiotics, talk with your doctor about the possible interactions.
Foods to Avoid With Levothyroxine
Other substances can cause effects similar to drug interactions. The authors of a July 2013 report in Thyroid listed several foods and beverages you should stay away from including grapefruit, grapefruit juice, soy protein and coffee. These products decrease the absorption and thus the effectiveness of levothyroxine. Fiber supplements may also negatively affect absorption of the thyroid medication.
Yet some food interactions can have beneficial effects. For example, getting abundant vitamin C increases levothyroxine's absorption, according to a February 2017 review in Clinical Therapeutics. Vitamin C is found in a variety of fruits and vegetables, including citrus fruits, strawberries, and bell peppers.
The writers of a June 2014 paper in the Journal of Clinical Endocrinology and Metabolism showed how vitamin C can help people with poor digestive absorption by testing 31 people with hypothyroidism and gastritis. The latter condition typically limits the digestive absorption of critical nutrients.
The study found that combining vitamin C and levothyroxine increased circulating levels of thyroid hormone, presumably due to improved absorption of the medication. Keep in mind that this was a small study, and further research is needed about the interactions between vitamin C and levothyroxine.
How to Take Levothyroxine
Taking levothyroxine with plain water on an empty stomach first thing in the morning will help you avoid most issues with substances that affect its absorption. Wait about an hour before you eat anything or drink coffee.
If you take iron or calcium supplements, the Mayo Clinic recommends taking them 4 hours apart from your levothyroxine. It's probably best to include these supplements with your lunch or dinner instead.
Always check with your health care provider or pharmacist about the proper way to take your medications, including levothyroxine.
Read more: Which Digestive Organ Absorbs Nutrients?
- Clinical Endocrinology: "Drugs That Interact With Levothyroxine"
- Lancet Diabetes and Endocrinology: "Treatment of Hypopituitarism in Patients Receiving Antiepileptic Drugs"
- Thyroid: "Serum Thyrotropin Levels Following Levothyroxine Administration at Breakfast"
- Clinical Therapeutics: "Factors Affecting Gastrointestinal Absorption of Levothyroxine"
- Journal of Clinical Endocrinology and Metabolism: "Effect of Vitamin C on the Absorption of Levothyroxine in Patients with Hypothyroidism and Gastritis"
- Thyroid: "Ciprofloxacin and Rifampin Have Opposite Effects on Levothyroxine Absorption"
- Mayo Clinic:"Levothyroxine (Oral Route)"