Understanding Reverse T3

A high reverse T3 is linked to nonthyroidal illness

Reverse T3 RT3 thyroid testing
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Do not feel alone if you have not heard of a blood test called reverse T3 (rT3), also known as reverse triiodothyronine. This is a blood test not commonly ordered by thyroid doctors, except in the setting of critical illness (imagine a person hospitalized in the intensive care unit). 

Let's take a look a closer look at this thyroid blood test, and the debate surrounding its true meaning.

What Is Reverse T3?

RT3 is a metabolite of T4 (thyroxine). Typically, when T4 loses an atom of iodine—a process known as monodeiodination, or T4 to T3 conversion—it becomes triiodothyronine (T3), the active thyroid hormone.

But in some cases, the body conserves energy by converting the T4 instead into rT3, an inactive form of T3 that is incapable of delivering oxygen and energy to the cells, as T3 does.

Measuring Reverse T3

RT3 can be measured by a simple blood test.

It's uncommonly ordered by doctors, except in the instance of seriously ill, hospitalized patients who are found to have abnormal thyroid function tests—usually a low or normal TSH, low triiodothyronine (T3) and possibly a low thyroxine (T4).

In order to best interpret these findings (determine whether the seriously critically ill person truly is hypothyroid or has a nonthyroidal illness syndrome), an rT3 is drawn.

Nonthyroidal Illness Syndrome 

Nonthyroidal illness, also known as euthyroid-sick syndrome, refers to the reduced level of thyroid function blood tests found in the setting of severe illness in a person without preexisting thyroid dysfunction. After recovery from the severe illness, the thyroid function test results should normalize.

In nonthyroidal illness syndrome, the rT3 is usually elevated, due to the body's suppression of certain enzyme activity. In central hypothyroidism, the rT3 is low.

With that, an elevated rT3 level in the setting of severe illness helps doctors exclude a diagnosis of hypothyroidism.

Due to the fact that in nonthyroidal illness, thyroid hormone reduction is considered a potentially protective mechanism (or possibly an artifact, according to some experts), doctors do not generally recommend treatment.

However, the decision on whether or not to treat nonthyroidal illness is still debated. Some experts believe that nonthyroidal illness syndrome indicates that tissue hypothyroidism is present. So, they believe that the person should be treated with thyroid hormone replacement (or even hypothalamic releasing factors) if thyroxine levels are below a certain level.

Moving forward, it's important to note that besides serious illness, other health conditions that may trigger nonthyroidal illness include: 

  • Starvation
  • Surgery
  • Bone marrow transplantation
  • Heart Attack
  • Coronary artery bypass grafting

Integrative Medicine's View on rT3

It's worthy to note that some integrative hormone experts have a different view of the role and value of rT3, believing that elevated levels of RT3 (even though TSH, Free T3, and Free T3 values may be within the normal reference range) may reflect a thyroid problem at the cellular level—a condition sometimes referred to as "cellular hypothyroidism."

In this integrative view, elevated rT3 can be triggered by ongoing chronic physical or emotional stress, adrenal fatigue, low ferritin (stored iron) levels, acute illness and injury, and chronic disease, among other factors.

According to integrative practitioners, one of the key ways to address elevated RT3 and "cellular hypothyroidism" is through thyroid treatment with a medication that contains T3.

A Word From Verywell

The take-home message is that the precise meaning or interpretation of rT3 is still being teased out. Hopefully, more research on the treatment (or not) of critically ill patients with high rT3 levels will be performed (as this has not yet been investigated in large, randomized studies). 

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