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Armour Thyroid vs. Synthroid - How do they compare?

Medically reviewed by Sally Chao, MD. Last updated on Sep 21, 2023.

Official answer


Armour Thyroid and Synthroid are both thyroid hormone replacement therapies.

  • Synthroid is a brand name for levothyroxine, a synthetic (man-made) form of the thyroid hormone thyroxine, or T4.
  • Armour Thyroid is a brand name for a natural form of thyroid hormone. It is made from the dried thyroid glands of pigs. It contains a combination of two thyroid hormones, triiodothyronine (T3) and thyroxine (T4).

Both medications come as tablets in many different strengths and are available by prescription only.

The very first form of thyroid hormone replacement to become available was made from drying and crushing thyroid glands from pigs. Later, scientists learned how to create synthetic versions of thyroid hormones. The early versions of the animal-based products resulted in variable potencies, and so these fell out of favor in the 1980s, but are still available today.

Levothyroxine, the synthetic form of T4, is the preferred drug according to current medical treatment guidelines. However, some patients who take levothyroxine still experience symptoms of hypothyroidism even when the hormone levels in their blood are within the desired range. For these patients, switching to a natural product that also contains T3, such as Armour Thyroid, may sometimes improve their symptoms.

Here is how these two products compare:

Armour Thyroid Synthroid
Contains Triiodothyronine (T3) and thyroxine (T4) Thyroxine (T4)
Source Pig thyroid glands Man-made
Formulation Tablet (in a variety of strengths) Tablet (in a variety of strengths)
Availability By prescription only By prescription only
Dosing Adjusted based on symptoms and lab monitoring Adjusted based on symptoms and lab monitoring
Monitoring Thyroid stimulating hormone (TSH), T4 levels

Symptom improvement

Thyroid stimulating hormone (TSH), T4 levels

Symptom improvement

Side effects Mainly signs of hyperthyroidism related to too much drug Mainly signs of hyperthyroidism related to too much drug
Duration of Treatment Lifetime Lifetime

The human thyroid gland, located just below the throat, makes and releases two thyroid hormones called triiodothyronine (T3) and thyroxine (T4). T4 is also converted to T3 in the bloodstream. These hormones work with other hormones and systems in the body to influence many bodily functions, including body temperature, heart rate, blood pressure and metabolism.

When your thyroid gland fails to make the proper amount of thyroid hormones, the disease is called hypothyroidism. There are many possible causes of hypothyroidism.

Some of the common symptoms of hypothyroidism include:

  • Constant fatigue
  • Weight gain and fluid retention
  • Constipation
  • Dry, itching skin
  • Dry, brittle hair and nails
  • Difficulty learning or remembering
  • Sensitivity to cold temperatures

People who have hypothyroidism will need to take a thyroid hormone replacement therapy for the rest of their lives. The effectiveness of this therapy is evaluated by monitoring your symptoms and by measuring the amount of T4 and another hormone called thyroid stimulating hormone (TSH) in the blood. The dose of thyroid hormone replacement may need to be adjusted occasionally in response to this monitoring.

Related Questions

  1. U.S. Food and Drug Administration (FDA). Synthroid. July 2020. Available at: [Accessed August 28, 2021].
  2. Allergan. Armour Thyroid. June 2018. Available at: [Accessed August 30, 2021].
  3. American Association of Clinical Endocrinology (AACE). Thyroid. Available at: [Accessed September 3, 2021].
  4. American Association of Clinical Endocrinology (AACE). Hypothyroidism. Available at: [Accessed August 27, 2021].
  5. Mateo RCI, Hennessey JV. Thyroxine and treatment of hypothyroidism: seven decades of experience. Endocrine. 2019;66(1):10-17.
  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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