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What is the difference between Levoxyl and Synthroid?

Medically reviewed by Sally Chao, MD. Last updated on Oct 7, 2021.

Official answer


Levoxyl and Synthroid are brand names for the drug levothyroxine. Levoxyl and Synthroid contain the same active ingredient, levothyroxine, but they contain different inactive ingredients.

Levoxyl tablets contain the inactive ingredients:

  • Calcium sulfate dihydrate
  • Croscarmellose sodium
  • Magnesium stearate
  • Microcrystalline cellulose
  • Sodium bicarbonate
  • Various color additives

Synthroid tablets contains the inactive ingredients:

  • Acacia
  • Confectioner’s sugar (contains corn starch)
  • Lactose monohydrate
  • Magnesium stearate
  • Povidone
  • Talc
  • Various color additives

You may have a reaction, called a hypersensitivity reaction, to some inactive ingredients, but the active ingredient is the same in both medications and is not known to cause reactions.

Therapeutic equivalents

The U.S. Food and Drug Administration (FDA) considers these two products to be therapeutically equivalent. Therapeutically equivalent drugs are expected to produce the same clinical effect and have the same safety profile. Because of this, therapeutically equivalent drugs may be legally substituted for one another by the pharmacist during the prescription filling process, unless the prescriber indicates he or she does not want any substitutions to be dispensed.

Even though the FDA has designated Levoxyl and Synthroid as therapeutic equivalents, the American Thyroid Association (ATA), in their most recently published guidelines, advises not to switch between levothyroxine products, regardless of brand name or generic status. Their concern is that such switching may result in enough variation to cause a noticeable effect on thyroid hormone levels and symptom control. The ATA notes that this is especially important for certain groups of individuals, including:

  • Pregnant women
  • Children
  • Those with high-risk thyroid cancer
  • Those who are frail

There are several brand name and generic levothyroxine products available on the market. All of them require a prescription from your doctor. Levoxyl, made by Pfizer, was first marketed in 2001, and Synthroid, made by AbbVie, was first marketed in 2002. They are both brand name products.

Each medication is available in many different tablet strengths. The highest strength of Levoxyl available is 200 micrograms, while the highest strength of Synthroid available is 300 micrograms.

Treating hypothyroidism

Levothyroxine is a synthetic (man-made) form of the thyroid hormone thyroxine, often referred to as T4. Levothyroxine is used to treat hypothyroidism in adults and children.

Hypothyroidism occurs when your thyroid gland fails to make enough thyroid hormone. The thyroid gland and the hormones it produces help control body temperature and metabolism and can affect other systems in the body, too. If you have hypothyroidism, you will need to take a thyroid hormone replacement therapy for the rest of your life.

The effectiveness of levothyroxine is evaluated by monitoring your symptoms and by measuring the amount of T4 and another hormone called thyroid stimulating hormone (TSH) in the blood. Your dose of levothyroxine may need to be adjusted in response to this monitoring.

It may take 4 to 6 weeks to see the full effect of a given dose of levothyroxine on symptoms and thyroid hormone levels. When levothyroxine is first started, the blood tests may be needed more often, about every 8 weeks. Once you are on a stable dose of levothyroxine, blood tests may only need to be done once or twice a year.

If a switch in levothyroxine products is necessary (for example, due to a drug shortage), the ATA guidelines recommend rechecking thyroid hormone levels once the full effect of the drug is expected. This would generally be in 4 to 6 weeks after starting the new product.

  1. U.S. Food and Drug Administration (FDA). Levoxyl. Last updated December 2018. Available at: [Accessed September 8, 2021].
  2. U.S. Food and Drug Administration (FDA). Synthroid. Last updated July 2020. Available at: [Accessed August 28, 2021].
  3. American Association of Clinical Endocrinology (AACE). Hypothyroidism. Available at: [Accessed August 27, 2021].
  4. U.S. Food and Drug Administration (FDA). Therapeutic equivalence. Last updated November 14, 2017. Available at: [Accessed September 17, 2021].
  5. American Thyroid Association (ATA). Guidelines for the Treatment of Hypothyroidism. Thyroid 24(12): 1670-1751, 2014.

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