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What Is Reverse T3 Thyroid Hormone?

Learn about reverse T3 (rT3) and the controversy surrounding this thyroid hormone.
What Is Reverse T3 Thyroid Hormone?

In this article:

  • Overview of thyroid hormones 
  • Production of rT3 
  • Clinical significance of rT3
  • Relationship to diet


The thyroid is a complex organ that helps control every cell in your body. Understanding how this small, but powerful gland works and what your lab tests mean can be challenging. Sometimes, your thyroid labs do not match up with how you are feeling. It is essential to understand what biomarkers to test and what the results mean for your overall health if you struggle with symptoms from a thyroid condition.


Overview of thyroid hormones


The thyroid gland is responsible for producing hormones that control the body's metabolism. Thyroid hormones regulate your metabolic rate by regulating your heart rate, blood pressure, body temperature, digestive system, muscle function, bone maintenance, and brain development. When your thyroid hormones are off, your whole body is affected. 


Creating and secreting thyroid hormones is a highly complex process. Indeed, there are many steps involved to make the right amount of thyroid hormone for your body. 


Thyroid hormone production begins in your brain. The pituitary gland, which is a pea-sized gland partially attached to the hypothalamus in your brain, is responsible for controlling most hormone-secreting glands in your body. Sometimes, even, the pituitary is called the "master gland" because it holds a significant role in regulating vital body functions and helps to control your overall wellbeing.


One of the main hormones released by the pituitary gland is thyroid-stimulating hormone (TSH). This hormone "stimulates" the thyroid gland to secrete thyroid hormones. Your doctor will test your TSH if you have any symptoms that may indicate a problem with your thyroid. 


  • High TSH levels may mean that you have an underactive thyroid or hypothyroidism. Your pituitary gland secretes more and more TSH to signal the thyroid gland to release more thyroid hormones because they are low.
  • Low TSH levels may suggest that you have an overactive thyroid or hyperthyroidism. Your thyroid is secreting too much thyroid hormone, so it releases less TSH to tell the thyroid to slow down.


The thyroid uses iodine from your diet to make two thyroid hormones: thyroxine (T4) and triiodothyronine (T3). When your thyroid works normally, it produces about 80% T4 and 20% T3. Although T4 is more abundant in your bloodstream, T3 is much more active. 


Production of rT3


Reverse T3 is the inactive metabolite of thyroxine (T4) formed by selective deiodination.


Deiodination is the process of converting T4 into T3 in some of your body tissues like the liver and kidneys. When T4 converts into T3, one iodine atom is removed from the outer ring of the T4 molecule. T4 is aptly named because it has four iodine atoms attached to tyrosine. Sometimes, one iodine atom is removed from the inner ring of the T4 molecule, which becomes reverse T3 (rT3).  


Reverse T3 is a byproduct of T4 metabolism and is the inactive form of T3. That is, rT3 cannot carry out the same metabolic activities as T3. RT3 is thought to elevate when a person is critically ill as a means of conserving precious energy.


Clinical significance of reverse T3


The clinical relevance of rT3 is hotly debated. Indeed, many questions surround what role rT3 plays and if there is any clinical significance to using this test for diagnosing thyroid disease.


There is no evidence that rT3 indicates thyroid illness. Still, many patients request to have their rT3 tested because they believe high rT3 levels are behind their hypothyroid symptoms such as fatigue and weight gain. 


That said, there are some circumstances where assessing rT3 can be useful. Reverse T3 may be an adaptive mechanism to benefit the body in certain situations where you don't want to have as much thyroid hormone activity in the tissue level. For example, if a patient is undergoing extreme stress, trauma, surgery, or malnutrition. Then, rT3 may be a valuable marker.


Another example is that it may be helpful to monitor rT3 levels in euthyroid sick patients (also called nonthyroidal illness syndrome). These are patients who have a healthy thyroid (euthyroid) and are hospitalized in the intensive care unit because they are critically ill. Typically, these patients are suffering from severe illness or have had major surgery, restricting calorie intake. RT3 is usually elevated in these patients, while T3 is low and T4 and TSH are normal. Treating euthyroid sick patients requires treating the underlying cause of severe illness and does not usually require thyroid medication. 


Reverse T3 relationship to diet


To reduce energy consumption, RT3 may be produced when a person is suffering from starvation. There is some speculation that rT3 may elevate when the body is under severe stress, such as in people who follow chronic diets.  


When a person follows strict diets that lead to calorie deprivation, it can stress the body and may increase rT3 levels. Consequently, people who diet to lose weight may retain or even gain weight because their body is attempting to store excess calories for energy reserves. The theory is that high rT3 levels may contribute to weight gain even in the presence of weight-loss efforts such as dieting and exercise. 


Many people with hypothyroidism struggle with weight gain and fatigue. Therefore, some patients request to have their rT3 levels checked to see if that is contributing to their symptoms. However, the significance of rT3 and what to do with laboratory findings remains unknown at this time. 


Current guidelines indicate that assessing your TSH, free T4, free T3, and TPO is sufficient for identifying thyroid disease and titrating your medication if you struggle with symptoms that may be related to a thyroid condition. 

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If you do check your rT3 and discover that it's elevated, keep in mind that it goes up in someone undergoing stress. It's important then to consider and treat the underlying contributors like stress or poor nutrition. 


A note from Paloma Health


Our thyroid panel was developed by our Chief Medical Officer to provide the most complete panel for most people. After a consultation, if your Paloma Health thyroid doctor believes there is a need for additional testing, including nutrient deficiencies or reverse T3, they can order those tests.  Above all, we are committed to staying current on all thyroid research and resources. We will update this article if studies indicate new information. 

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