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What is the difference between NP Thyroid and Armour Thyroid?

Medically reviewed by Sally Chao, MD. Last updated on Oct 1, 2021.

Official answer


NP Thyroid and Armour Thyroid are two different brands of natural thyroid hormone tablets.

Both products are made using dried ground thyroid glands from pigs and contain a combination of two thyroid hormones, triiodothyronine (T3 liothyronine) and tetraiodothyronine (T4 levothyroxine) in the same proportion. Some of the inactive ingredients in the two medicines are different.

NP Thyroid contains the following inactive ingredients:

  • Calcium stearate
  • Dextrose monohydrate
  • Maltodextrin
  • Mineral oil

Armour Thyroid contains the following inactive ingredients:

  • Calcium stearate
  • Dextrose
  • Microcrystalline cellulose
  • Sodium starch glycolate
  • Opadry white

Who makes NP Thyroid and Armour Thyroid?

NP Thyroid is manufactured by Acella Pharmaceuticals.

Armour Thyroid is manufactured by Allergan Pharmaceuticals.

Both medicines are available by prescription only.

Natural ​vs. synthetic thyroid replacement therapy

There are two main categories of thyroid replacement therapy:

  • Natural products, which are made from grinding up dried thyroid glands from animals
  • Synthetic (man-made) products, which are made in a laboratory

Within each category, there are several different products available.

The natural products have been used as replacement therapy since the 1890s, even before the U.S. Food and Drug Administration (FDA) and its regulations existed. Many years later, scientists learned how to create synthetic versions of T3 and T4.

One challenge with natural products has been the variability in their potency because the amount of hormone in an animal's thyroid gland may vary. This has resulted in reports of failed response from too little thyroid hormone or side effects from too much thyroid hormone, as well as drug recalls of some natural products over the years.

As a result of the difficulties with variability in potency and the availability of synthetic forms of thyroid hormone, the natural thyroid products fell out of favor in the 1980s, but they may still be prescribed today and benefit some patients.

None of the current natural thyroid products on the market are approved by the FDA.

Levothyroxine, the synthetic form of T4, is the preferred drug for thyroid hormone replacement, according to current treatment guidelines. However, some people who take levothyroxine still experience symptoms of hypothyroidism even when the hormone levels in their blood are within the desired range. For these people, switching to a natural product, such as NP Thyroid or Armour Thyroid, may help improve symptoms.

T3 and T4

The human thyroid gland makes and releases two thyroid hormones called triiodothyronine (T3) and thyroxine (T4). T4 is also converted to T3 in the bloodstream.

The thyroid gland has an important role in regulating your body temperature and metabolism. When your thyroid gland fails to make enough thyroid hormones, the disease is called hypothyroidism. If you have hypothyroidism, you will need to take thyroid hormone replacement for the rest of your life.

The effectiveness of thyroid replacement therapy is measured by monitoring your symptoms and by checking the amount of two main thyroid-related hormones, thyroid stimulating hormone (thyrotropin) and T4, in your blood. The full effects of thyroid replacement therapy may take weeks to months to realize.

  1. Acella Pharmaceuticals. NP Thyroid. Last revised January 2019. [Accessed September 17, 2021].
  2. Allergan. Armour Thyroid. Last updated June 2018. Available at: [Accessed August 30, 2021].
  3. American Association of Clinical Endocrinology (AACE). Hypothyroidism. Available at: [Accessed August 27, 2021].
  4. Mateo RCI, Hennessey JV. Thyroxine and treatment of hypothyroidism: seven decades of experience. Endocrine. 2019;66(1):10-17.
  5. U.S. Food and Drug Administration (FDA). Thyroid hormone replacement. June 14, 2021. Available at: [Accessed September 10, 2021].
  6. Ettleson MD, Bianco AC. Individualized Therapy for Hypothyroidism: Is T4 Enough for Everyone? J Clin Endocrinol Metab. 2020;105(9):e3090-e3104.

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