The key difference between thyroxine and triiodothyronine is that thyroxine contains four atoms of iodine per molecule while triiodothyronine contains three atoms of iodine per molecule.
The thyroid gland is an endocrine gland located in the neck area. This gland is important since it produces three hormones. Two of these hormones are essential for regulating metabolism. These two thyroid hormones are thyroxine and triiodothyronine. Both hormones function together in order to control the rate of energy use in our body. The third thyroid hormone is calcitonin and it is important in maintaining calcium homeostasis.
CONTENTS
1. Overview and Key Difference
2. What is Thyroxine
3. What is Triiodothyronine
4. Similarities Between Thyroxine and Triiodothyronine
5. Side by Side Comparison – Thyroxine vs Triiodothyronine Neuron in Tabular Form
6. Summary
What is Thyroxine?
Thyroxine is a thyroid hormone which contains four atoms of iodine per molecule. This hormone is primarily responsible for the regulation of metabolism. It is a tyrosine hormone and has iodine in the molecule. Thyroxine is the major form of thyroid hormone present in the blood.
Figure 01: Thyroxine
Furthermore, thyroxine accounts for 80% of the total hormonal production by the thyroid gland. In addition, whole thyroxin originates from thyroid secretion, unlike triiodothyronine. Moreover, thyroxine has a longer half-life than triiodothyronine.
What is Triiodothyronine?
Triiodothyronine is one of the two major thyroid hormones released by our thyroid gland. Most triiodothyronine in our blood exists as a bound form with proteins. Some amount remains as an unbound form. When measuring the total triiodothyronine, it gives the total amount that circulates in the bloodstream. Normal reference range of the total triiodothyronine is 80 – 200 ng/dL. Below and above this range reflects an abnormality in secreting thyroid hormone and a functional problem of our thyroid gland.
Figure 02: Triiodothyronine
When the total triiodothyronine level is high, we call this condition hyperthyroidism while when it is below, we call it hypothyroidism. Free triiodothyronine is a small percentage which is in the unbound form with proteins. Normal reference range of free triiodothyronine in our bloodstream is 2.3- 4.2 pg/mL. This level represents the immediately available triiodothyronine hormone which can be used. Hence, it is considered that free triiodothyronine is the best representation of the patient’s hormonal status. Furthermore, free triiodothyronine levels are important for the differential diagnosis of hyperthyroidism and non-thyroidal illnesses.
Triiodothyronine plays several vital roles in our body. It regulates the metabolic rate. It also regulates the heart and digestive functions, brain development and function, muscle and bones, etc.
What are the Similarities Between Thyroxine and Triiodothyronine?
- Thyroxine and triiodothyronine are two hormones synthesized and released by the thyroid gland.
- They are tyrosine-based hormones.
- Furthermore, they are primarily responsible for the regulation of metabolism in our body.
- Production of both hormones decreases due to iodine deficiency.
- Both thyroxine and triiodothyronine are used to treat thyroid hormone deficiency (hypothyroidism).
- These hormones bind with thyroid hormone receptors in order to act.
- Moreover, they travel with the bloodstream.
What is the Difference Between Thyroxine and Triiodothyronine?
Thyroxine and triiodothyronine are two thyroid hormones. However, thyroxine has four iodine atoms per molecule while triiodothyronine has three iodine atoms per molecule. Thus, this is the key difference between thyroxine and triiodothyronine. Furthermore, whole thyroxine originates from the thyroid secretion while most triiodothyronine originates from the deiodination of thyroxine.
The below infographic summarizes the difference between thyroxine and triiodothyronine.
Summary – Thyroxine vs Triiodothyronine
Thyroxine and triiodothyronine are two major hormones produced and released by the thyroid gland. However, the thyroid gland produces more thyroxine than triiodothyronine. The key difference between thyroxine and triiodothyronine is that thyroxine has four iodine atoms per molecule while triiodothyronine has three iodine atoms per molecule. Furthermore, whole thyroxine originates from the thyroid secretion while most triiodothyronine originates from thyroxine deiodination. This is the summary of the difference between thyroxine and triiodothyronine.
Reference:
1. “Thyroid Hormones.” Wikipedia, Wikimedia Foundation, 25 Aug. 2019, Available here.
Image Courtesy:
1. “Levothyroxine2DCSD” By Brenton (talk) – Own work (Public Domain) via Commons Wikimedia
2. “Liothyronine2DCSD” By Brenton (talk) – Own work (Public Domain) via Commons Wikimedia