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Does levothyroxine cause weight gain or loss?

Medically reviewed by Sally Chao, MD. Last updated on Sep 3, 2021.

Official answer

by Drugs.com

In most cases, levothyroxine causes some weight loss. According to the American Thyroid Association, when this medication is started, you may lose up to 10% of your weight. This weight is mainly water weight, since being hypothyroid makes you retain water.

Levothyroxine is a medication to treat hypothyroidism. Since weight gain is one of the symptoms of hypothyroidism, taking thyroid replacement with levothyroxine may reverse that weight gain. You may return to the normal body weight you would have been without the fluid retention caused by hypothyroidism.

Once your thyroid levels are normal again, this medication should have no effect on your weight.

Levothyroxine should not be used to treat obesity or for weight loss. This can cause dangerous side effects, especially if you combine levothyroxine with another drug that causes weight loss, such as dextroamphetamine or methamphetamine.

Side effects of levothyroxine to watch for and contact your doctor about include:

  • Nausea
  • Vomiting
  • Headache
  • Diarrhea
  • Joint pain or muscle cramps
  • Fever
  • Increased heat sensitivity
  • Appetite changes
  • Menstrual cycle changes in women.

Taking too much levothyroxine can cause overdose symptoms that include:

  • A racing heart
  • Chest pain
  • Shortness of breath
  • Anxiety
  • Irritability
  • Tremors
  • Insomnia
  • Sweating.

Levothyroxine is an oral medication that comes in a capsule or tablet. You take it once each day on an empty stomach, usually before breakfast. It may take several weeks for your thyroid levels to return to normal.

Symptoms of hypothyroidism that may be reversed with levothyroxine include:

  • Lack of energy
  • Constipation
  • Skin that’s thick and dry
  • Fatigue
  • Increased cold sensitivity
  • Hair loss
  • Pain in your joints and muscles
  • Irregular menstrual periods in women
  • Depression.

You should not stop taking levothyroxine on your own, even if you feel better. This can result in a return of hypothyroidism.

References
  1. American Thyroid Association (ATA). Thyroid and Weight. Available at: https://www.thyroid.org/thyroid-and-weight/. [Accessed August 9, 2021].
  2. U.S. National Library of Medicine MedlinePlus. Levothyroxine. July 2021. Available at: https://medlineplus.gov/druginfo/meds/a682461.html. [Accessed August 9, 2021].

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