What is the Difference Between T3 and Free T3

The main difference between T3 and free T3 is that T3 or triiodothyronine is one of the two hormones secreted by the thyroid gland, whereas the free T3 is the unbound form of triiodothyronine hormone found in the serum. Furthermore, the protein-bound form of T3 is the circulating form of T3 while the free T3 is the active form of the hormone.  

In overall, T3 and free T3 are two forms of the thyroid hormone, triiodothyronine secreted by the thyroid gland.  

Key Areas Covered 

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

Key Terms 

Free T3, Pituitary Gland, Serum Proteins, Thyroid Hormone, T3

What is T3 

T3 or triiodothyronine is one of the two types of thyroid hormones, while the second one is T4 or thyroxine. Generally, both of them are responsible for the regulation of growth and development, metabolism, body temperature, and heart rate. Moreover, T4 is the prohormone of T3. TSH or thyroid-stimulating hormone regulates the production and secretion of T4. Basically, T4 contains four iodine atoms and is converted into T3 by the removal of a single iodine atom in a process called deiodination, which primarily occurs in the liver and kidney.

Figure 1: Thyroid System

Furthermore, T3 is roughly four times more potent than T4. However, the direct production of T3 is 20% of the total thyroid hormone production, while the rest is T4. Approximately, 85% of T3 is the converted form of T3 from T4. Moreover, T3 concentration in the serum is around one-fortieth of the T4 concentration. Besides, the half-life of T3 in the blood is about 2.5 days while it is 6.5 days for T4. 

What is Free T3 

Free T3 is the non-protein-bound form of triiodothyronine hormone in the serum. Generally, both T3 and T4 are bound to plasma proteins to be transported through the blood. This binding significantly increases the half-life of the hormone in the blood. In contrast, it decreases the activity of the hormone by limiting the rate of the intake of the hormone by peripheral tissues. Furthermore, there are three main types of serum proteins, which bind to both T3 and T4. 

Figure 2: Deiodination

Basically, they include thyroxine-binding globulin (TBG), transthyretin (TTR), and serum albumin. However, both TBG and TTR are glycoproteins with a higher affinity to T4. On the contrary, serum albumin has a higher capacity to bind to T3 and T4 due to its higher availability. But, this binding also has a lower affinity. Moreover, only 0.5% of T3 remains free in the serum. They are the active form of T3 as well. Moreover, the unbound form of T3 can be estimated by the triiodothyronine resin uptake test by determining the saturation of binding spots on TBG.  

Similarities Between T3 and Free T3 

• T3 and free T3 are two forms of triiodothyronine hormone, which is a secretion of the thyroid gland. 
• They are responsible for the regulation of metabolism, body temperature, and heart rate. 
• Moreover, their abnormal levels are the indicators of hypothyroidism, hyperthyroidism, and slow metabolism. 

Difference Between T3 and Free T3 

Definition 

T3 refers to a thyroid hormone, which plays an important role in the regulation of metabolism while free T3 refers to the unbound form of triiodothyronine hormone, which is the active form. 

Binding to Proteins 

While T3 is usually the protein-bound form, free T3 is not bound to proteins. 

Significance 

T3 is the circulatory form of triiodothyronine while free T3 is the active form of triiodothyronine. 

Percentage 

Generally, 80% of T3 is bound to TBG, 5% to TTR, and 15% to albumin and lipoproteins while 0.5% of T3 is free in the serum. 

Conclusion 

T3 or triiodothyronine hormone is one of the two types of hormones secreted by the thyroid gland. Generally, most of the T3 is bound to proteins in the serum. Therefore, they are inactive and are in the circulating form. Significantly, a higher proportion of T3 is protein-bound. On the other hand, free T3 is the unbound form of triiodothyronine hormone in the serum. However, it is the active form of triiodothyronine and is responsible for the regulation of metabolism, body temperature, and heart rate along with the other thyroid hormone, T4. However, a very low proportion of T3 is free. Hence, the main difference between T3 and free T3 is their binding to proteins, activity, and their proportions.  

References:

1. “What Is Thyroid T3 and FT3: Difference between T3 and FT3.” KlinicApp, 31 May 2018, Available Here.
2. “T3 And T4 – What’s the Difference?” Nursing Blog | Lippincott NursingCenter | T3 and T4 – What’s the Difference?, Available Here.

Image Courtesy:

1. “Thyroid system” By Mikael Häggström (Public Domain) via Commons Wikimedia   
2. “Iodothyronine deiodinase” By Excession~commonswiki assumed (based on copyright claims). Own work assumed (based on copyright claims). (CC BY-SA 3.0) via Commons Wikimedia