Levothyroxine: food and drug interactions to know about

Levothyroxine (or levothyroxine sodium) helps millions of people manage their hypothyroidism (underactive thyroid gland). You may be hypothyroid because of an autoimmune disease (like Hashimoto's thyroiditis), thyroid surgery, radiation therapy, or just having a low functioning thyroid. 

Levothyroxine is a synthetic form of a thyroid hormone called T4 and acts as a thyroid hormone replacement therapy for low thyroid hormone levels. In combination with other treatments, levothyroxine is also sometimes used to treat certain forms of thyroid cancer (DailyMed, 2019). Levothyroxine is also available under the brand names Synthroid, Levothroid, Unithroid, Tirosint, and Levoxyl.

Getting just the right level of thyroid replacement hormone circulating throughout the body is key to helping you feel and function well. Unfortunately, given other medicines and substances people take, this can be a bit of a balancing act.  

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Interactions can lead to problems in two main ways. Some interactions with medicines or supplements increase the concentration of levothyroxine in your body, leading to an increased risk of side effects. 

Alternatively, certain drugs, foods, or dietary supplements can get in the way of how your body absorbs and uses levothyroxine—this decreases the hormone therapy's effectiveness and leaves you feeling the symptoms of low thyroid hormone levels. Much of the interference occurs in your upper intestine, where 40% to 80% of levothyroxine is absorbed (DailyMed, 2019). Taking it with other medications may require your provider to adjust your dose of levothyroxine.

Levothyroxine can also make other medications not work as well or increase their risk of adverse events. Your best option is to seek the medical advice of your healthcare provider or pharmacist to help navigate the drug information and potential interactions. Make them aware of any changes in your medications, diet, supplements, and medical diagnoses. 

What medications should you avoid with levothyroxine?

Prescription drugs and over-the-counter medications that may interact with levothyroxine include DailyMed, 2019):  

• Antacids with calcium carbonate (like over-the-counter Tums) can prevent levothyroxine from being absorbed; take calcium carbonate at least four hours apart from your levothyroxine.

• Antacids with magnesium and aluminum (like over-the-counter Maalox, Milk of Magnesia, Mylanta) may interfere with levothyroxine absorption by lowering stomach acid levels.

• Proton pump inhibitors (PPIs) like omeprazole (brand name Prilosec), lansoprazole (brand name Prevacid), pantoprazole (brand name Protonix), and esomeprazole (brand name Nexium) lower stomach acid levels and can interfere with levothyroxine absorption.

• Bile acid sequestrants (like colesevelam, cholestyramine, colestipol, sevelamer), used to lower cholesterol, may prevent levothyroxine from being absorbed; take these medications at least four hours apart from your levothyroxine.

• Sucralfate, a prescription medication used to treat stomach ulcers, may decrease levothyroxine absorption.

• Rifampin, an antibiotic, increases the breakdown of levothyroxine, making it less effective.

• Blood thinners (anticoagulants like warfarin), when taken with levothyroxine, are more likely to cause abnormal bleeding. 

• Sympathomimetic drugs (albuterol, dopamine, ephedrine, etc.) have an increased risk of adverse events when taken with levothyroxine, especially in people with heart disease.

• Tyrosine-kinase inhibitors, a type of cancer medication, may decrease how well levothyroxine works.

• Diabetic medications (like insulin, metformin, etc.) and digoxin (a heart pill) can be less effective if taken with levothyroxine.

• Ketamine and levothyroxine increase your risk of significantly high blood pressure (hypertension) and fast heart rate (tachycardia) when taken together.

• Orlistat, a weight loss medication, may bind to levothyroxine and prevent its absorption (Filippatos, 2008).

• Antidepressants and levothyroxine have varying interactions depending on the type of antidepressant. For example, taking tricyclic antidepressants (like amitriptyline) with levothyroxine increases the risk of toxicity (abnormal heartbeats or nervous system problems) of both medications. Alternatively, the antidepressant sertraline makes levothyroxine less effective. Talk to your healthcare provider if you plan on starting antidepressants while on levothyroxine therapy.

This may not include all possible drug interactions, and you should seek medical advice from your healthcare provider or pharmacist for additional drug information.

What supplements should you avoid with levothyroxine?

Some over-the-counter dietary supplements can interact with levothyroxine, including (DailyMed 2019):

• Ferrous sulfate or iron supplement: Don't take within four hours of thyroid hormone medications as it can prevent normal absorption of levothyroxine.

• Calcium supplements: These can interfere with the absorption of levothyroxine; don't take within four hours of thyroid medications.  

Does biotin affect thyroid levels?

Biotin is a member of the vitamin B family, vitamin B7, that is marketed as a dietary supplement for several conditions, including to strengthen skin and hair. However, biotin can interfere with thyroid blood test results. 

According to the American Thyroid Association, biotin can make your thyroid function blood tests, like your T4 or thyroid-stimulating hormone (TSH) levels, appear abnormal when your thyroid hormone levels are actually normal in your bloodstream. To avoid this effect, wait at least two days after stopping biotin before you have your thyroid hormone levels checked (American Thyroid Association, n.d.). 

Medications are not the only things that can affect levothyroxine's effectiveness—certain food interactions can also occur, leading to a decrease in the absorption of levothyroxine, including (UpToDate, n.d.):

• Soybean flour

• Soy

• Walnuts

• Dietary fiber

• Cottonseed meal

• Espresso coffee

• Grapefruit

For optimal results, take levothyroxine on an empty stomach first thing in the morning, at least 30 to 60 minutes before breakfast. The goal is to be consistent from day to day so that you can avoid ups and downs in your thyroid levels over time. The same consistency is important if you want to take the levothyroxine at night; take it three hours after your last meal and avoid taking it along with other medicines (Jonklaas, 2014). 

Talk to your healthcare provider or pharmacist about any other interaction concerns.

Levothyroxine is generally well tolerated, with most of the side effects related to getting too much hormone replacement and experiencing symptoms of high thyroid hormone levels (hyperthyroidism). However, the U.S. Food and Drug Administration (FDA) has a black box warning for thyroid hormones, like levothyroxine. You should not use them for weight loss or to treat obesity. Large doses of levothyroxine can lead to serious and life-threatening effects (DailyMed, 2019).

Common side effects of levothyroxine include (DailyMed, 2019):

• Hair loss

• Weight loss

• Increased appetite

• Inability to tolerate high temperatures (heat intolerance)

• Fever

• Excessive sweating (hyperhidrosis)

• Diarrhea

• Muscle tremors

• Nervousness/anxiety

• Tiredness

• Decreased bone mineral density

• Irregular menstrual cycles

• Fertility problems

People with certain medical conditions may need to avoid using thyroxine or use it with caution because of the risk of more severe adverse effects, like rapid heartbeat, high blood pressure, and other heart problems. These medical conditions include, but are not limited to (DailyMed, 2019):

• Diabetes because levothyroxine can interfere with your blood sugar medications

• Osteoporosis because levothyroxine can worsen the bone loss

• Heart disease (or cardiovascular disease) because of the increased risk of heart problems

Discuss your other medical conditions with your healthcare provider before starting levothyroxine.

Several things can interact with levothyroxine, from prescription medicines to foods. But as long as you keep talking to your healthcare providers—and are open about any medications, herbs, or dietary supplements you take—you should be able to keep your levothyroxine levels steady to help you feel your best.