What is the Difference Between T3 and Reverse T3

Thyroid hormones play an important role in regulating the body’s metabolism, growth, and development. T3 and reverse T3 are two forms of thyroid hormones that play distinct roles in the body’s metabolism.

What is the difference between T3 and reverse T3? T3 or triiodothyronine is the active form of thyroid hormone, whereas the Reverse T3 (rT3) is an inactive form of T3.

Key Areas Covered

1. What is T3 (Triiodothyronine)  
      – Definition, Features
2. What is Reverse T3  
      – Definition, Features 
3. Similarities Between T3 and Reverse T3
      – Outline of Common Features
4. Difference Between T3 and Reverse T3
      – Comparison of Key Differences
5. FAQ: T3 and Reverse T3
      – Answers to Frequently Asked Questions

Key Terms

T3, Triiodothyronine, rT3, Reverse T3

What is T3 (Triiodothyronine)?

Triiodothyronine, commonly called T3, is the active form of thyroid hormone. The thyroid gland releases two primary hormones: thyroxine (T4) and T3. While T4 is the predominant hormone the thyroid gland produces, it is mainly inactive. T3, on the other hand, is the active form and is responsible for exerting the metabolic effects of thyroid hormones. T3 has a relatively short half-life of about 24 to 36 hours, necessitating its continuous production and conversion from T4 to maintain appropriate levels in the bloodstream.

The conversion of T4 to T3 occurs primarily in peripheral tissues like the liver and kidneys. Enzymes called deiodinases catalyze this process. Once converted, T3 can enter cells and bind to nuclear receptors, influencing gene expression and modulating metabolic activity.

T3 is essential for several important functions in the body. It helps regulate metabolism by increasing the amount of oxygen and energy cells use, which boosts the basal metabolic rate. T3 also plays a key role in growth and development. Additionally, T3 influences the function of various neuroendocrine tissues, which impacts mood, cognition, and overall neurological health.

What is Reverse T3 (rT3)?

Reverse T3 (rT3) is an inactive form of T3. Like T3, rT3 is derived from T4. However, instead of being converted by removing an iodine atom from the outer ring, rT3 is produced when an iodine atom is removed from the inner ring of T4. This process results in 3,3′,5′-triiodothyronine, a mirror image of T3 that lacks metabolic activity. Unlike T3, rT3 does not activate thyroid hormone receptors and does not contribute to metabolic processes.

The production of rT3 occurs primarily in peripheral tissues and is often a response to physiological stress. By reducing the conversion of T4 to T3, the body lowers its overall metabolic rate, preserving vital energy stores. Conditions such as severe illness, starvation, or trauma can increase the conversion of T4 to rT3, which leads to higher levels of this inactive hormone. Normal levels of rT3 are generally below 250 pg/ml. Elevated rT3 levels can be a diagnostic marker that indicates underlying health issues such as chronic stress, acute illness, or nutritional deficiencies.

Similarities Between T3 and Reverse T3

1. T3 and rT3 are derived from T4, the primary hormone produced by the thyroid gland.
2. Both hormones are iodinated derivatives of thyroxine, with slight structural differences determining their activity.
3. The production and conversion of T4 to either T3 or rT3 are regulated by the same enzymatic processes, influenced by the body’s metabolic needs and external stressors.

Difference Between T3 and Reverse T3

Definition

• T3 or triiodothyronine is the active form of thyroid hormone, whereas reverse T3 (rT3) is an inactive form of T3.

Active vs Inactive

• T3 is metabolically active, and it influences cellular metabolism, while rT3 is inactive.

Production

• T3 is produced by the removal of an iodine atom from the outer ring of T4, whereas rT3 is produced by the removal of an iodine atom from the inner ring, which produces a slightly different structure (Rt3 -3,3′,5′-Triiodothyronine).

Function

• T3 levels are maintained to support normal metabolic functions, while rT3 levels increase in response to stress, illness, or starvation to decrease metabolic activity.

Metabolism

• T3 increases metabolism while rT3 inhibits metabolism.

Conclusion

T3 and reverse T3 are two forms of thyroid hormones that play distinct roles in the body’s metabolism. The basic difference between T3 and reverse T3 is that T3 or triiodothyronine is the active form of thyroid hormone, whereas the Reverse T3 (rT3) is an inactive form of T3.

FAQ: T3 and rT3

1. What is triiodothyronine?

Triiodothyronine is the most active thyroid hormone in the body. It is also called T3 and is produced by the thyroid gland and from the conversion of another hormone. T3 levels impact whether you feel energetic or lazy.

2. Is T3 bad for the liver?

T3 itself isn’t directly harmful to the liver. It actually plays a role in healthy liver function.  However, very high doses or underlying liver issues can be a concern.

3. Does T3 increase cholesterol?

No, T3 does not increase cholesterol. T3 (thyroid hormone) helps the body process cholesterol. However, low T3 levels can lead to higher cholesterol.

4. What happens when T3 levels decrease?

When T3 levels are low, the body slows down. This can lead to fatigue, feeling cold, and weight gain. This might result in hypothyroidism, which is a treatable condition.

Reference:

1. “Thyroid Hormone.” Cleveland Clinic.
2. “Overview of Reverse T3 Thyroid Hormone.” Very Well Health.

Image Courtesy:

1. “Thyroid hormone synthesis” By Häggström, Mikael (2014). “Medical gallery of Mikael Häggström 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 2002-4436 (Public Domain) via Commons Wikimedia 
2. “Reverse triiodothyronine” By LegionMammal978 (talk) – Own work using: ChemDraw (Public Domain) via Commons Wikimedia