Reverse T3 (rT3)
An inactive mirror-image form of the thyroid hormone T3. The body makes more of it when unwell, starving or stressed, diverting thyroid hormone away from its active form. It is mainly a marker of serious non-thyroid illness rather than a thyroid test in its own right.
What it measures
The thyroid gland's main hormone, T4, is converted in the tissues into two products: active T3, which drives the body's metabolism, and reverse T3 (rT3), an inactive, mirror-image molecule that does not switch metabolism on. The balance between them is a way the body dials its metabolic rate up or down. Reverse T3 rises when the body wants to conserve energy — during serious illness, injury, surgery, starvation, prolonged fasting and major stress. In these situations, called non-thyroidal illness or 'euthyroid sick syndrome', the body shunts T4 towards inactive rT3 and away from active T3, effectively slowing metabolism while the thyroid gland itself is working normally. So a high reverse T3 usually signals that a person is significantly unwell or under metabolic stress, not that the thyroid is diseased. This is why standard thyroid assessment relies on TSH, free T4 and free T3 rather than rT3. Reverse T3 is sometimes ordered to help interpret confusing thyroid results in an ill patient, or is promoted in wellness settings as a measure of 'thyroid conversion' — a use that is not well supported and can be misleading if read outside its proper context.
What a high value can mean
- Serious illness, injury or surgery — the commonest reason; the body diverts thyroid hormone to the inactive form (non-thyroidal illness).
- Starvation, prolonged fasting or very low-calorie dieting.
- Major physical or physiological stress.
- Certain medications — such as high-dose steroids or some heart drugs.
- Interpreted with TSH, free T4 and free T3 — rT3 alone rarely changes management.
What a low value can mean
- Normal or low — the expected result in a healthy person; not usually a concern.
When to discuss with a doctor
Reverse T3 is a niche test, not part of a standard thyroid check, which relies on TSH, free T4 and free T3. Its clearest use is in a seriously ill patient with confusing thyroid results, where a high rT3 with a low T3 supports non-thyroidal illness — the pattern of a normal thyroid responding to severe illness — rather than true thyroid disease, sparing unnecessary treatment. It is often promoted in wellness and 'adrenal fatigue' settings as a measure of thyroid conversion, but this use is not well supported and a raised rT3 in that context usually just reflects stress, dieting or illness. A truly abnormal thyroid picture is defined by TSH, free T4 and free T3. Mediora.AI shows reverse T3 alongside the standard thyroid markers so it is read in context and not over-interpreted; the assessment belongs with your doctor.