What is the Difference Between Levothyroxine and Thyroxine

Levothyroxine and thyroxine are related terms. Thyroxine is a hormone. Hormones are chemical messengers made by glands that travel through blood to organs. When hormone levels go off-balance, it can lead to conditions like hypothyroidism or diabetes. In these instances, people use synthetic hormone replacements.

What is the difference between levothyroxine and thyroxine? Levothyroxine is a synthetic version of thyroxine.

Key Areas Covered

1. What is Levothyroxine 
      – Definition, Features
2. What is Thyroxine
      – Definition, Features 
3. Similarities Between Levothyroxine and Thyroxine
      – Outline of Common Features
4. Difference Between Levothyroxine and Thyroxine
      – Comparison of Key Differences
5. FAQ: Levothyroxine and Thyroxine
      – Answers to Frequently Asked Questions

Key Terms

Levothyroxine, Thyroxine, T3, T4, Thyroid

Difference Between Levothyroxine and Thyroxine

What is Levothyroxine

Levothyroxine is a synthetic form of the T4 hormone that is biochemically and physiologically indistinguishable from the natural hormone. Levothyroxine is useful clinically in conditions like primary, secondary, or tertiary hypothyroidism, where the body cannot produce adequate thyroxine. It is an oral medication. It is absorbed mostly in the duodenum, jejunum, and ileum. Levothyroxine has a bioavailability of 60-80%.


Levothyroxine has the same mechanism of action as the natural thyroxine hormone following absorption. The main metabolic route for T4 involves deiodination reactions, where deiodinase enzymes remove iodine. This process converts T4 to the more active T3 or to inactive reverse T3 (rT3). Moreover, both natural thyroxine and levothyroxine are highly bound to proteins, and their volume distribution is limited.

What is Thyroxine

Thyroxine is a hormone secreted by the thyroid gland. It is regulated by the hypothalamic-pituitary-thyroid (HPT) axis. It is also known as T4. The release of thyroid-stimulating hormone (TSH) from the pituitary is stimulated by thyrotropin-releasing hormone (TRH) released by the hypothalamus. T3 and T4 are synthesized and released from the follicular cells following the stimulation by TSH. Once in circulation, T4 and T3 exert a negative feedback mechanism on the production of TRH and TSH to regulate their own levels. Both T3 and T4 are highly bound to plasma proteins helping the maintenance of T4 and T3 stability and half-lives.


Thyroxine and T3 enter cells by active transport. In peripheral tissues, deiodinases convert T4 into more potent T3. Deiodinase enzyme removes iodine from T4.

Thyroxine has a major role in metabolic actions, such as body temperature and heart rate, and growth and development, such as brain development and overall growth, especially in children.

Similarities Between Thyroxine and Levothyroxine

  • Both act by entering cells and converting to T3 to exert their physiological effects.
  • Both thyroxine and synthetic T4 (levothyroxine) are highly bound to plasma proteins, aiding in their stability and functionality in the bloodstream.

Difference Between Levothyroxine and Thyroxine


  • Thyroxine is a hormone secreted by the thyroid gland. Levothyroxine is a synthetic form of the T4 hormone that is biochemically and physiologically indistinguishable from the natural hormone.


  • Thyroxine is natural and produced by the body, while levothyroxine is man-made.


  • Levothyroxine is specifically used as a treatment for hypothyroidism to replace deficient thyroid hormones, whereas thyroxine naturally maintains normal metabolic functions in the body.


  • Levothyroxine is administered orally, and its absorption can be affected by various factors, including food and other medications. In contrast, natural thyroxine is endogenously produced and regulated within.


Both levothyroxine and thyroxine are chemically identical and work the same way in the body. Levothyroxine is a medication that treats hypothyroidism, while thyroxine is naturally available in the body. Basically, levothyroxine is a synthetic version of thyroxine. This is the basic difference between levothyroxine and thyroxine.

FAQ: Levothyroxine and Thyroxine

1. Are levothyroxine and thyroxine the same?

Levothyroxine and thyroxine are very similar. Thyroxine (T4) is a natural hormone produced by the thyroid gland, while levothyroxine is a manufactured medication, a synthetic version of T4.

2. What are the two types of thyroxine?

There aren’t actually two types of thyroxine, but the thyroid gland produces two main thyroid hormones: thyroxine (T4) and triiodothyronine (T3).

3. What is another name for thyroxine?

Tetraiodothyronine (T4) is another name for thyroxine. This term reflects the four iodine atoms present in the molecule’s structure.

4. What is the greatest risk of levothyroxine?

The greatest risk of levothyroxine isn’t necessarily the medication itself but rather taking an incorrect dosage. Taking too much levothyroxine can lead to a condition called hyperthyroidism. Not taking enough levothyroxine can worsen hypothyroidism symptoms.

5. What happens if thyroxine is high?

If your thyroxine (T4) is high, it can indicate a condition called thyrotoxicosis. This means there’s too much thyroid hormone circulating in your body. Symptoms of this condition include an irregular heartbeat, unexplained weight loss, rapid heartbeat, and muscle weakness. 


1. “Thyroid Hormone.” Cleveland Clinic.
2. Colucci P, Yue CS, Ducharme M, Benvenga S. “A Review of the Pharmacokinetics of Levothyroxine for the Treatment of Hypothyroidism.” Eur Endocrinol. 

Image Courtesy:

1. “Levothyroxine 200” By Emeldir (Public Domain) via Commons Wikimedia
2. “Thyroid system” By Mikael Häggström – (Public Domain) via Commons Wikimedia

About the Author: Hasini A

Hasini is a graduate of Applied Science with a strong background in forestry, environmental science, chemistry, and management science. She is an amateur photographer with a keen interest in exploring the wonders of nature and science.

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