Difference Between T3 and T4

The main difference between T3 and T4 is that the T3 refers to the active thyroid hormone whereas the T4 refers to the precursor of the thyroid hormone produced by the thyroid gland. Furthermore, T4 is converted into T3 by an enzyme called iodothyronine deiodinase, mainly in the liver.

Therefore, T3 and T4 are the two forms of thyroid hormone that regulates the metabolism. T3 is known as triiodothyronine while T4 is known as thyroxine.

Key Areas Covered

1. What is T3
     – Definition, Formation, Effect
2. What is T4
     – Definition, Formation, Effect
3. What are the Similarities Between T3 and T4
     – Outline of Common Features
4. What is the Difference Between T3 and T4
     – Comparison of Key Differences

Key Terms

Formation, Regulation of Metabolism, T3, T4, Thyroid Gland

What is T3

T3 (triiodothyronine) is the active form of the thyroid hormone that can readily go into the cells of the body. That means every cell of the body has thyroid receptors. Further, out of the T3 hormones produced by the thyroid gland, 80% is in the form of T4, and the rest of the 20% is directly in the form of T3. TSH (thyroid-stimulating hormone) is the pituitary hormone, which stimulates the thyroid gland for the production of T4.

Figure 1: Thyroid Hormone Synthesis

What is T4

T4 (thyroxine) is the prohormone of T3. Iodothyronine deiodinase is the enzyme responsible for the conversion of T4 into T3. This conversion mainly occurs inside the liver but, every cell of the body contains this enzyme as well.

Figure 2: Formation of T3

The effects of the thyroid hormones on the body are as follow.

• Increases the cardiac output, heart rate, ventilation rate, and basal metabolic rate
• Enhances the effects of catecholamines (i.e. increases sympathetic activity)
• Increases brain development
• Thickens endometrium in females
• Increases catabolism of carbohydrates and proteins

On the other side, if the thyroid hormones produced are insufficient, the body may slow down the metabolism, giving rise to symptoms of hypothyroidism. An excess of circulating free thyroid hormones causes hyperthyroidism.

Similarities Between T3 and T4

• T3 and T4 are two types of thyroid hormones which can regulate metabolism.
• Iodine and tyrosine are the precursors of thyroid hormones.
• The thyroid gland produces both the hormones.
• These hormones then pass into the bloodstream and get transported throughout the entire body to regulate the metabolism.
• The transportation of around 95% of the thyroid hormones through the blood is by binding to the plasma proteins.
• Both have the same effect on the body.
• They regulate growth and development, metabolism, body temperature, and heart rate.
• The use of both is to treat hypothyroidism.

Difference Between T3 and T4

Definition

T3 refers to a thyroid hormone that affects almost every physiological process in the body while T4 refers to the main hormone produced by the thyroid gland.

Known as

The T3 is known as triiodothyronine while T4 is known as thyroxine.

Production/Formation

Formation of most of the T3 hormone is from T4 hormone in the liver while the production of the T4 hormone is in the thyroid gland.

Precursors

T3 is mainly formed from the diiodo-tyrosine (DID) and monoiodo-tyrosine (MIT) while T3 is formed by combining two DIDs.

Amounts Produced by the Thyroid Gland

The thyroid gland produces less T3 but more T4.

Activity

T3 is the active form of the thyroid hormone while T4 is the inactive form.

Potency

The T3 is five times more potent than the T4.

Normal Levels in the Blood

Total T3 in the blood should be 5.0-12 μg/dL, and the free T4 should be 80-190 ng/dL while total T4 in the blood should be 1.0-3.0 ng/dL and free T3 should be 0.25-0.65 ng/dL.

Duration of Action

T3 has a shorter duration of action while T4 has a longer duration of action.

Half-Life

The half-life of T3 is about one day while the half-life of T4 is about seven days.

Synthetic Forms

Liothyronine is the synthetic form of T3 while levothyroxine is a synthetic form of T4.

Medical Uses

The use of T3 is to treat myxoedema coma while the use of T4 is to treat myxoedema coma and for the regular treatment of myxoedema.

Conclusion

Of the two thyroid hormones produced by the thyroid gland, the T3 is the active form and has the highest potency while the T4 is the inactive form and is less potent. But both hormones help in the regulation of the metabolism. Therefore, the main difference between T3 and T4 is their activity.

Reference:

1. “Triiodothyronine.” You and Your Hormones, Available Here
2. “Thyroxine.” You and Your Hormones, Available Here

Image Courtesy:

1. “Thyroid hormone synthesis” By Mikael Häggström, Own work (CC0) via Commons Wikimedia
2. “Iodothyronine deiodinase”  (CC BY-SA 3.0) via Commons Wikimedia

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