If you have thyroid disease, you may have come across contradictory or confusing information about reverse T3 (rT3), also known as reverse triiodothyronine. The blood test for this thyroid hormone is controversial because, while most research does not point to a reliable way to interpret or treat abnormal results, there has been some interest in determining whether rT3 levels could be of value in understanding thyroid disease or severe medical illnesses.

RT3 is a blood test that not commonly ordered, but if your doctor tested you for it and you are interested in understanding what your results mean, it's worth exploring the debate surrounding it.

An Overview of Thyroid Hormones

There are a number of thyroid hormones. The most commonly measured ones are thyroid stimulating hormone (TSH), total thyroxine (T4), free T4, total triiodothyronine (T3), and free T3.

TSH: TSH is released from the pituitary gland in the brain, and it stimulates the thyroid gland to make thyroid hormones.
T4 and T3: The thyroid gland makes and releases T4, as well as some T3. Total T3 and T4 levels include T3 and T4 that is bound to protein as well as T3 and T4 that is not (known as free T3 and T4).
T4 conversion to T3 and rT3: After its release from the thyroid gland, T4 is converted to T3, which is an active thyroid hormone, or to rT3, which is considered an inactive form. The rate and ratio of T4 conversion to either T3 or rT3 depend on the body's metabolic needs.

Thyroid Function Tests and Normal Ranges

You can have hyperthyroidism or hypothyroidism due to a problem with any step in the process of thyroid hormone stimulation, production, activation, or protein binding. Your doctors look at these tests to determine which step or steps are responsible for your symptoms.

Production of rT3

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

The body also converts T4 into rT3, which is 3,3´5´-triiodothyronine, an inactive form of T3 that is incapable of the metabolic activity that is normally carried out by T3.

It is believed that the body produces rT3 in times of severe illness or starvation as a mechanism of preserving energy.

Measuring rT3

RT3 can be measured by a blood test. A level below 250 pg/ml (10 to 24 ng/dL) is considered normal.

Possible Significance

There is some understanding of the relevance of rT3, but it is not clear whether the results would direct any specific treatment plan.

It appears that it can be elevated in times when the body is undergoing a crisis situation. Some evidence also points to a possible reparative effect of rT3 on the body.

Nonthyroidal Illness Syndrome

In some instances, people who are seriously ill, often in the intensive care unit of a hospital, may have a low or normal TSH with low T3 and possibly a low T4 level as a result of critical illness, even without having thyroid disease.

Nonthyroidal illness, also known as euthyroid-sick syndrome, refers to a 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.

Nonthyroidal Illness

People who are hypothyroid may have a low rT3, while people who are critically ill can have a high rT3. However, these changes do not always occur, so most critical care experts do not rely on rT3 to help distinguish thyroid disease from other causes of major illness.

Furthermore, since thyroid hormone reduction is considered a potentially protective mechanism in nonthyroidal illness, doctors do not generally recommend treatment based on rT3 alone. Instead, a number of well-understood physical signs and laboratory tests are used to guide these decisions.

Other Health Conditions

Other health conditions that may trigger changes in thyroid test results, including rT3, without thyroid disease include:

Starvation
Surgery
Bone marrow transplantation
Heart Attack
Coronary artery bypass grafting

RT3 and Dieting

Some experts suggest that rT3 levels could also be raised by chronic dieting, resulting in difficulties with losing weight as the body preserves excess calories and energy unnecessarily.

This idea has also not been consistently proven, and the treatment is also not clear.

A Word From Verywell

The take-home message is that the precise meaning and significance of rT3 are still being teased out. At the current time, there are no standardized guidelines with respect to interpretation or treatment planning based on rT3 alone. Rest assured, however, that TSH, T4, free T4, T3, and free T3 levels are adequate indicators of your thyroid disease severity and management.