Thyroid Hormones (T3 and T4): What it is, and How are They Different?

Thyroid Hormones (T3 and T4): What it is, and How are They Different?

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The thyroid is an endocrine gland situated in the front of your neck involved in the production and secretion of thyroid hormones: triiodothyronine (T3) and thyroxine (T4). These hormones have similarities in their composition and function. So, they are collectively called thyroid hormones. However, there are significant differences that exist between T3 and T4. So, we also have to study them separately.

T3 and T4 regulate the body's metabolism, and the disorders associated with the thyroid gland are due to too little or too much thyroid hormone secretion in the blood. Although the disorder is more common in women, men are also affected by the disease associated with thyroid function.

In this article, we intend to describe the thyroid hormones T3 and T4. We will also discuss their functions and the difference between these two hormones. We also try to highlight the conditions associated with the abnormality of thyroid hormone levels and what to do if you have abnormal thyroid hormone levels.

What is thyroid hormone?

The thyroid hormone is the hormone that regulates the rate of metabolism in the body. Metabolism can be defined as the process by which the body breaks down food products and produces energy for consumption.

The thyroid gland produces the hormone. The thyroid gland is a butterfly-shaped gland located in the front part of the neck, under the skin.

The thyroid hormone represents two different hormones. Triiodothyronine (T3) and thyroxine (T4) are two different hormones.

How are T3 and T4 different?

How are T3 and T4 different?

T3 and T4 are collectively called thyroid hormones and are produced by the thyroid gland. T3 has three iodine molecules, and T4 has 4 iodine molecules.

T3 and T4 differ in many different ways. Some differences between T3 and T4 are mentioned below:


T3 is named triiodothyronine, while T4 is called thyroxine.


The thyroid gland produces both T3 and T4. 20% of the thyroid hormone produced by the gland is T3, and 80% is in the form of T4. The T4 produced by the thyroid gland is converted to T3 in the peripheral tissue.

T4 is created by combining two diiodo-tyrosines (DIDs), whereas T3 is primarily created by combining MIT and a diiodo-tyrosine (DID).


T3 is around five times more potent than T4. However, the activity of T4 lasts longer, with the half-life of T3 vs. T4 being one day and seven days, respectively. As T3 is a more potent and active form, the continuous production of T3 is ensured by the longer half-life of T4 and peripheral conversion of T4 to T3.

Normal Level in blood

The normal range of total T3 in the blood is 5-12 microgram/dl, whereas the normal range of T4 is 1-3 nanogram/dl.

Synthetic forms and their functions

The synthetic form of T3 is Liothyronine, and the same for T4 is Levothyroxine. 

Liothyronine is used for the treatment of myxoedema coma. Myxedema coma is a severe condition due to decreased thyroid hormone, leading to reduced mental status, hypothermia, and other symptoms related to slowing function in multiple organs.

Levothyroxine is used for the treatment of hypothyroidism.

What is the function of the thyroid hormone?

After releasing hormones into the bloodstream, they move to the target cells. The receptors of those cells are better able to use T3 than T4. So, T3 is called the active form, and T4 is the inactive form.

T3 and T4 hormones act on each cell in the body and carry out the following functions:

What is the function of the thyroid hormone?

  • Regulates the speed at which energy is consumed by each cell. This regulation helps balance body weight and determines weight gain or loss, called the metabolic rate. So, the hormone regulates basal metabolic rate (BMR).
  • Regulates the heart rate.
  • Regulates body temperature.
  • Regulates the movement and digestion of food in the digestive tract.
  • In children, the hormone is responsible for brain development.
  • Controls muscle contractions.
  • Controls the normal process of replacement of dead cells.

How are thyroid hormone levels regulated?

The regulation of the thyroid hormone level involves organs such as the hypothalamus, pituitary gland, thyroid gland, and various other hormones.

The hypothalamus is the part of the brain, and the pituitary gland is the small pea-sized gland located in the base of the brain below the hypothalamus.

The hypothalamus releases a hormone called thyroid releasing hormone (TRH), which stimulates the pituitary gland for the secretion of a hormone called thyroid stimulating hormone (TSH). TSH stimulates the thyroid gland for the production of T3 and T4. 

A feedback loop controls this hormonal chain. When T3 and T4 levels increase in the blood, it decreases TRH and TSH release, which in turn decreases T3 and T4 production.

Similarly, when T3 and T4 levels decrease, this feedback is turned off, resulting in TRH and TSH release, leading to increased T3 and T4 production.

The disorder in the thyroid hormone level is associated with the hypothalamus, pituitary gland, or thyroid gland. As any of these organs are affected, it affects the feedback loop and thus the production of T3 and T4.

Thyroid function tests

Thyroid function tests

The thyroid function tests measure the levels of different thyroid hormones and hormones regulating thyroid gland function. The common hormone levels measured in the blood are:

  • Total T4 Level
  • Total T3 Level
  • Free T4 Level
  • Free T3 Level
  • Thyroid-stimulating hormone (TSH) level.

Besides thyroid function tests, your health care provider might also order further tests to diagnose thyroid-related disorders.

  • Thyroid antibodies: These tests help to diagnose autoimmune conditions affecting the thyroid gland.

  • Thyroglobulin: It is a protein found in the thyroid gland. These levels help to diagnose thyroiditis or thyroid inflammation and to monitor the treatment of thyroid cancer.

  • Thyroid scan: It is an imaging procedure that helps study the thyroid gland's structure and function.

Conditions associated with the abnormality of thyroid hormone levels

Thyroid disorder is one of the common endocrine disorders with rising prevalence. The condition is five to eight times more common in females than in males.

Some of the conditions associated with the abnormality of thyroid hormone levels are:

  • Hypothyroidism is a condition in which the thyroid hormone is lower than the normal level in the blood. It is also called "underactive thyroid," with a variety of symptoms. Some common symptoms of hypothyroidism are weight gain, feeling tired, and a decreased ability to tolerate cold temperatures.

  • Hyperthyroidism is also called "overactive thyroid," in which the thyroid hormone level is higher than the normal range. The condition can involve any organ system, with symptoms varying from person to person. However, some common symptoms are unintentional weight loss, unable to tolerate hot temperatures, and an irregular or fast heartbeat.

  • Hashimoto thyroiditis: an autoimmune condition that causes hypothyroidism

  • Graves' disease: an autoimmune condition that causes hyperthyroidism

  • Thyroiditis: inflammation of the thyroid gland. 

  • Goiter: an enlarged thyroid gland. Can be associated with hypothyroidism or hyperthyroidism.

  • Thyroid cancer.

What to expect if I had abnormal thyroid hormone levels?

If you have an abnormal level of thyroid hormones along with the symptoms suggestive of thyroid disorder, your healthcare provider will plan for treatment according to the disorder.

For an underactive thyroid or hypothyroidism, synthetic forms of T3 and T4 might be prescribed. Depending on the condition associated, you might have to take medicine for a certain period or for life.

For overactive thyroid or hyperthyroidism, treatment can be offered in various ways:

  • Medicines to control symptoms such as beta blockers to control heart rate.

  • Treatment to reduce thyroid hormone synthesis, such as antithyroid drugs, radioiodine, or surgery.


Thyroid hormones are endocrine hormones produced by the thyroid gland. There are two forms of thyroid hormone: triiodothyronine (T3) and thyroxine (T4). T3 is considered an active form, and T4 is a non-active form of thyroid hormone. T4 is converted to T3 in peripheral tissues. There are many other similarities and differences between these two hormones, which are important for diagnosing various thyroid disorders.

The thyroid hormone regulates the metabolism in each cell and organ of our body and the disorders of the thyroid gland are associated with symptoms differing from person to person. Some common symptoms are associated with weight changes, being unable to tolerate temperature changes, changes in heart rate, and gastrointestinal symptoms. These changes depend on whether one is suffering from an overactive thyroid or an underactive thyroid. Hypothyroidism is a more common condition than hyperthyroidism.

You need to see your healthcare provider when you have symptoms suggestive of a thyroid hormone abnormality. They might do certain thyroid function tests to determine the abnormality's cause.

Once the cause of the condition is identified, the treatment varies according to the condition you are suffering from and the cause of the condition. The good news is that most diseases associated with thyroid gland dysfunction are highly treatable.

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Author: Dr. Sony S. | Panel Expert, Medical Doctor Dr. Sony is known for her medical articles, written with in-depth detail and accuracy owing to her vast medical knowledge and thorough research of each article. She completed her degree with multiple scholarships from Guangzhou Medical University and is a board-certified Clinical Doctor. She is currently working as a Medical Officer in the emergency department of a renowned hospital and continues to publish numerous medical papers and articles. Dr. Sony continues to lead the way in medical breakthroughs, unparalleled by her high level of detail, knowledge and passion for discovering new sciences and innovative healthcare treatments.

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