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What is the difference between Unithroid and Synthroid?

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

Official answer


Unithroid and Synthroid are brand names for the drug levothyroxine. Both medications contain the same active ingredient, levothyroxine, but some of their inactive ingredients are different.

Unithroid contains the inactive ingredients:

  • Acacia
  • Colloidal silicon dioxide
  • Corn starch
  • Lactose
  • Magnesium stearate
  • Microcrystalline cellulose
  • Sodium starch glycolate
  • Various color additives

Synthroid contains the inactive ingredients:

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

You may experience a reaction (known as a hypersensitivity reaction) to some inactive ingredients, but the active ingredient is the same and not known to cause reactions.

Therapeutic equivalents

The U.S. Food and Drug Administration (FDA) considers these two medications to be therapeutically equivalent. Therapeutically equivalent drugs are presumed to both 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 when the prescription is being filled, unless the prescriber indicates that he or she does not want any substitutions to be dispensed.

Even though the FDA has designated Unithroid and Synthroid as therapeutic equivalents, the American Thyroid Association (ATA), in their latest published guidelines, advises not to switch between levothyroxine products, regardless of the brand name or generic status. The ATA’s 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 to consider for certain groups of individuals, such as:

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

There are several brand name and generic levothyroxine products available on the market, and they all require a prescription from your doctor.

Unithroid and Synthroid are both brand name products. Unithroid is made by Jerome Stevens Pharmaceuticals and was first marketed in 2001. Synthroid is made by AbbVie and was first marketed in 2002. Each medication is available in many different tablet strengths.

Treating hypothyroidism

Levothyroxine is a synthetic (man-made) form of the thyroid hormone thyroxine, which is often referred to as T4. Levothyroxine is used to treat hypothyroidism in both 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 important body functions such as body temperature and metabolism. Thyroid hormones can affect other systems in the body as well. People who have hypothyroidism need to take a thyroid hormone replacement therapy for the rest of their lives.

The effectiveness of levothyroxine is evaluated by monitoring your symptoms and by measuring the amount of T4 and another hormone known as thyroid stimulating hormone (TSH) in the blood. The levothyroxine dose 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, roughly every 8 weeks. Once you’re on a stable dose of levothyroxine, blood tests may only need to be performed once or twice a year.

If a switch in levothyroxine products is necessary (such as if there’s a shortage of your drug), the ATA guidelines recommend rechecking thyroid hormone levels once the full effect of the drug is expected. Generally, this would be in 4 to 6 weeks after starting the new medicine.

  1. U.S. Food and Drug Administration (FDA). Unithroid. Last updated December 2018. Available at: [Accessed September 10, 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. 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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