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

box of synthroid medication

The Bottom Line

Levothyroxine is a prescription medication that is used to treat hypothyroidism (underactive thyroid activity). Levothyroxine works by mimicking the activity of the natural hormone that is normally generated by the thyroid gland. Levothyroxine is a medication that is generally taken for life to maintain normal thyroid function.

doctor palpates patient's thyroid

What is levothyroxine used for?

Levothyroxine is an oral medication used to treat a condition called hypothyroidism, or underactive thyroid gland activity. The thyroid is a major gland in the body that is involved in important functions such as metabolism, growth, development, and regulating body temperature. Levothyroxine is a synthetic version of the main hormone of the thyroid gland, known as thyroxine or 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 function.  Common brand names of levothyroxine in the United States 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 may 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 may interfere with the absorption of levothyroxine. The most common medications that interact with levothyroxine are aluminum and magnesium-containing antacids, calcium carbonate, iron, cholestyramine, sucralfate, and sevelamer. To reduce the risk of drug interactions, it’s recommended to take levothyroxine at least 4 hours before taking the medications mentioned above. 

Consumption of certain foods and beverages such as soybean flour, cotton seed meal, walnuts, grapefruit juice, and dietary fiber may decrease the body’s absorption of levothyroxine. Coffee may also decrease the absorption of levothyroxine. These interactions can be avoided if levothyroxine is taken at least 30 minutes to an hour before consuming any of these foods and beverages. 

Levothyroxine is unlikely to interact with alcohol. It’s generally safe to drink alcohol while on this medication 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 may fluctuate during pregnancy, a doctor may need to adjust the dose 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 may 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 may also be present illicitly in some over-the-counter dietary supplements. These dietary supplements may contain varying amounts of thyroid hormones, which can be dangerous and cause unwanted adverse effects. Because of this, people should not take over-the-counter weight loss supplements that contain thyroid hormones. 

What happens if you take too much levothyroxine? 

Levothyroxine toxicity is rare, but can sometimes occur accidentally, especially in children. If someone takes too much levothyroxine, symptoms may not occur for several days. Symptoms of levothyroxine overdose may include an 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 minutes to an hour 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 on accident, 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.

Synthroid® is a registered trademark of ABBVIE INC.

Levoxyl® is a registered trademark of Pfizer, Inc.

Unithroid® is a registered trademark of Jerome Stevens Pharmaceuticals, Inc.

Tirosint® is a registered trademark of IBSA Institute Biochimique.

Larissa Razo, BS, PharmD Candidate 2023
Student Pharmacist

Poisoned?

Call 1-800-222-1222 or

HELP ME online

Prevention Tips

  • Keep levothyroxine out of reach of children.
  • Store levothyroxine away from heat, light, and moisture.
  • Always take levothyroxine exactly as prescribed by your doctor.

This Really Happened

A 29-year-old female ingested 2.5mg (100 tablets of 25 mcg) of levothyroxine. She developed symptoms of confusion, mild headache, and tingling sensations in her body. She was taken to an Emergency Department and was admitted to the intensive care unit for observation. Her blood thyroxine (T4) level increased after the ingestion, but went down in the following days, and she was able to be discharged from the hospital 2 days after the ingestion. 

References

Eghtedari B, Correa R. Levothyroxine. [Updated 2022 Sep 4]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK539808/

Hypothyroidism in pregnancy. American Thyroid Association. (n.d.). Retrieved December 8, 2022, from https://www.thyroid.org/hypothyroidism-in-pregnancy/ 

Kiran Kumar KC, Ghimire N, Limbu T, Khapung R. Levothyroxine overdose in a hypothyroid patient with adjustment disorder: A case report. Ann Med Surg (Lond). 2020 Oct 9;59:234-236. doi: 10.1016/j.amsu.2020.09.045. PMID: 33088498; PMCID: PMC7566947.

Llanyee Liwanpo, Jerome M. Hershman, Conditions and drugs interfering with thyroxine absorption, Best Practice & Research Clinical Endocrinology & Metabolism, Volume 23, Issue 6, 2009, Pages 781-792, ISSN 1521-690X, https://doi.org/10.1016/j.beem.2009.06.006.

Synthroid (levothyroxine) tablets [prescribing information]. North Chicago, IL: AbbVie Inc; July 2020.

Wiesner A, Gajewska D, Paśko P. Levothyroxine Interactions with Food and Dietary Supplements-A Systematic Review. Pharmaceuticals (Basel). 2021 Mar 2;14(3):206. doi: 10.3390/ph14030206. PMID: 33801406; PMCID: PMC8002057.

Poisoned?

Call 1-800-222-1222 or

HELP ME online

Prevention Tips

  • Keep levothyroxine out of reach of children.
  • Store levothyroxine away from heat, light, and moisture.
  • Always take levothyroxine exactly as prescribed by your doctor.

This Really Happened

A 29-year-old female ingested 2.5mg (100 tablets of 25 mcg) of levothyroxine. She developed symptoms of confusion, mild headache, and tingling sensations in her body. She was taken to an Emergency Department and was admitted to the intensive care unit for observation. Her blood thyroxine (T4) level increased after the ingestion, but went down in the following days, and she was able to be discharged from the hospital 2 days after the ingestion.