Reverse T3 Test
Reverse T3 (rT3) is an inactive form of thyroid hormone produced when the body diverts T4 away from making active T3. It rises during illness, starvation and stress.
The test is used by some clinicians but remains controversial, and it is not part of standard thyroid assessment.
Last medically reviewed: 15 November 2025
What it measures
T4 can be converted either to active T3 or to inactive reverse T3. When the body is under stress or unwell, it favours reverse T3, effectively conserving energy. A high reverse T3 with low T3 can reflect this shift.
Because reverse T3 is not a direct measure of thyroid function, its value is limited.
Why it is controversial
Reverse T3 levels fluctuate with illness, dieting and many medications, making them hard to interpret. Mainstream guidelines do not recommend it for diagnosing or managing thyroid disease.
Some practitioners use the ratio of free T3 to reverse T3 to support a diagnosis of “tissue resistance,” but this approach is not supported by strong evidence.
Practical advice
If you are considering a reverse T3 test, discuss it with a clinician who can interpret it in context. It should not replace standard tests such as TSH and free T4.
Frequently asked questions
Should I have a reverse T3 test?
It is not part of standard thyroid testing and is not recommended by mainstream guidelines. Discuss whether it adds anything to your situation with a specialist.
What does a high reverse T3 mean?
It often reflects illness, stress or dieting rather than thyroid disease, which is why it is difficult to interpret.
Can it diagnose thyroid resistance?
The evidence for using reverse T3 to diagnose tissue resistance is weak, and the test is not a reliable basis for treatment decisions.
References & further reading
This page is based on established medical knowledge and guidelines from authoritative sources. Always consult a qualified clinician for personal medical advice.
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