Synthroid (Levothyroxine): Side effects, Interactions, and Overdose

What is levothyroxine used for?

Levothyroxine is an oral medication used to treat underactive thyroid gland activity (hypothyroidism). The thyroid is a major gland in the body that is involved in important functions such as metabolism, growth, development, and regulation of body temperature. Levothyroxine is a synthetic version of the main hormone of the thyroid gland, known as thyroxine (T4). In patients who have an underactive thyroid, there is not enough production of thyroid hormones and levothyroxine is prescribed to help maintain normal thyroid hormone function. Common brand names of levothyroxine in the US include Synthroid, Levoxyl, Unithroid, and Tirosint.

What are the side effects of levothyroxine?

Some common side effects of levothyroxine include fast heart rate, insomnia, fatigue, hair loss, and weight loss. Side effects can happen in the first months after starting levothyroxine until the body adjusts to it.

Levothyroxine interactions: What foods and medications should be avoided while taking levothyroxine?

Certain foods and medications can interfere with the absorption of levothyroxine. The most common medications are aluminum- and magnesium-containing antacids, calcium carbonate, iron, cholestyramine, sucralfate, and sevelamer. To reduce the risk of impaired absorption, it’s recommended to take levothyroxine at least 4 hours before taking any of the medications listed above.

Consumption of certain foods and beverages such as soybean flour, cotton seed meal, walnuts, grapefruit juice, coffee, and dietary fiber can decrease the absorption of levothyroxine. These interactions can be avoided if levothyroxine is taken at least 30–60 minutes before consuming any of these foods and beverages. It’s generally safe to drink alcohol as long as the alcohol is not consumed at the same time as the levothyroxine dose.

Can I take levothyroxine while pregnant?

Levothyroxine is safe to use during pregnancy. Currently available studies have not demonstrated increased risk to the fetus in pregnant women taking levothyroxine. Because thyroid function can fluctuate during pregnancy, a doctor might need to adjust the dosage of levothyroxine taken during pregnancy. 

Should I take levothyroxine for weight loss?

Thyroid hormones can increase the body’s metabolism and promote weight loss. Some people might use their levothyroxine prescriptions for this reason, but it’s important to know that the use of levothyroxine for weight loss can result in dangerous and potentially life-threatening side effects. Thyroid hormones can also be present illicitly in some over-the-counter weight loss products. These dietary supplements can contain varying amounts of thyroid hormones, which can be dangerous and cause adverse effects. Because of this, people should not take over-the-counter weight loss supplements.

What happens if you take too much levothyroxine? 

Levothyroxine toxicity is rare, but it can sometimes occur accidentally, especially in children. If someone takes too much levothyroxine, symptoms might not occur for several days. Symptoms of levothyroxine overdose include irregular heartbeat, headache, anxiety, agitation, shaking, fatigue, confusion, and disorientation.

How should you take levothyroxine? 

Levothyroxine is best taken on an empty stomach in the morning. Take levothyroxine 30–60 minutes before breakfast.

What to do if someone takes too much levothyroxine or takes it by accident 

If someone takes too much levothyroxine or takes it accidentally, get guidance from Poison Control immediately. Help from Poison Control is available online at www.poison.org and by phone at 1-800-222-1222. Both options are free, confidential, and available 24 hours a day.

Larissa Razo, BS, PharmD Candidate 2023
Student Pharmacist