The key difference between T3 and Free T3 is that the T3 is the bound form of T3 with proteins while the free T3 is the unbound form of T3.
The thyroid gland is one of the organs in our body. It is located at the base of our neck and produces several hormones that are necessary to control our metabolism. Triiodothyronine (T3) and thyroxine (T4) are two thyroid hormones. Both T3 and T4 together help to control our metabolism or the rate of energy use in our body. Therefore, among T3 and T4, T3 is the strongest and causes the effects of thyroid hormones. T3 exists as a bound form with proteins. On the other hand, a small percentage is not bound with proteins, and they are known as free T3.
CONTENTS
1. Overview and Key Difference
2. What is T3
3. What is Free T3
4. Similarities Between T3 and Free T3
5. Side by Side Comparison – T3 vs Free T3 in Tabular Form
6. Summary
What is T3?
Triiodothyronine or T3 is a thyroid hormone released by our thyroid gland. Most of the T3 in our blood exists as a bound form with proteins. Total T3 refers to the collection of both bound and unbound forms of T3. Therefore, when measuring the total T3, it gives the total amount of T3 that circulates in the bloodstream.
Figure 01: Thyroid System
The normal reference range of the total T3 is 80 – 200 ng/dL. Below and above this range reflects an abnormality in secreting thyroid hormone and the functional problem of our thyroid gland. When the total T3 level is high, it is the condition called hyperthyroidism while when it is below, it is the hypothyroidism.
What is Free T3?
Free T3 is the form of T3 that is unbound with proteins. Compared to total T3 in our bloodstream, Free T3 is in a small percentage. However, measuring free T3 is more accurate than measuring total T3.
The normal reference range of free T3 in our bloodstream is 2.3- 4.2 pg/mL. Free T3 level represents the immediately available thyroid hormone which can be used. Hence, it is considered that free T3 is the best representation of the patient’s hormonal status. Furthermore, free T3 levels are important for differential diagnosis of hyperthyroidism and non-thyroidal illnesses.
What are the Similarities Between T3 and Free T3?
- Both are forms of T3 hormone.
- Both T3 and Free T3 can be tested to find out thyroid gland disorders.
What is the Difference Between T3 and Free T3?
T3 is a thyroid hormone produced by our thyroid gland, and T3 and free T3 are two forms. T3 is the bonded form with proteins while free T3 is the unbound form of T3. However, most of the T3 exists bound to proteins while free T3 present in a small percentage. Free T3 is readily available to use, and the test that measures free T3 gives an accurate idea about the patient’s hormonal state.
Summary – T3 vs Free T3
T3 is a thyroid hormone that is present in our bloodstream. Most of them exist bonded with proteins while a small percentage is present unbounded. The unbounded form is known as free T3 which is readily available for use. Free T3 and total T3 tests evaluate the function of the thyroid gland whether it is functioning well or not. This is the difference between T3 and Free T3.
Reference:
1.“Triiodothyronine.” Wikipedia, Wikimedia Foundation, 19 July 2018. Available here
2.“T3 Test: Purpose, Procedure & Risks.” Healthline, Healthline Media. Available here
Image Courtesy:
1.’Thyroid system’By Mikael Häggström (Public Domain) via Commons Wikimedia