Reverse T3 (rT3) test explained

Price: from €35; Medicina practica (2026-09-19)
Updated: 2026-09-10; Every number with its source

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What reverse T3 is

Reverse T3 (rT3) is a hormone whose structure is very similar to triiodothyronine (T3), but the iodine atoms sit in different positions, which makes the molecule biologically inactive. The body produces it when thyroxine (T4) in peripheral tissues is converted not into active T3 but into this inactive form. According to the American Thyroid Association, a certain amount of rT3 is produced normally but is broken down quickly, and in healthy, outpatient patients measuring this marker does not answer the question of whether hypothyroidism is present.

Why it is not a standard thyroid test

For that reason rT3 is not considered clinically useful and is not among the thyroid function tests recommended as standard. For routine assessment of thyroid function doctors rely on TSH, free T4 (FT4) and, when needed, free T3 (FT3) or total T3. People most often ask about rT3 after coming across it in the context of alternative medicine or information found online, where it is linked to unexplained fatigue or to so-called thyroid hormone resistance — concepts that lack scientific grounding.

Reverse T3 and ordinary T3 side by side

Feature	T3	Reverse T3 (rT3)
Biological activity	The active form of the hormone — it acts on cell receptors and regulates metabolism	Biologically inactive and broken down quickly in the body
Structure	Triiodothyronine — the active hormone molecule	The same chemical formula as T3, but the iodine atoms sit in different positions
How it forms	Thyroxine (T4) is converted into active T3 in peripheral tissues	In the tissues T4 is converted not into active T3 but into this inactive form
In routine thyroid assessment	Free or total T3 is used when needed, alongside TSH and free T4	Not among the thyroid function tests recommended as standard

When rT3 is tested

When a patient or a doctor wants an additional way to assess unexplained chronic fatigue while the standard thyroid tests (TSH, free T4) are normal — although, according to the ATA, in healthy, non-hospitalised people the rT3 test does not help establish whether hypothyroidism is present and is not clinically useful
In rare cases, in a specialist or research setting, when assessing T4 metabolism in seriously ill, hospitalised patients
When a patient asks for the test themselves, having read about rT3 online or in functional medicine sources
When the T3 to rT3 ratio is compared for scientific or research purposes — such tests are rarely done in ordinary clinical practice

How to prepare

No special preparation is usually needed — an ordinary venous blood sample is enough for the test
It is worth discussing the medicines you take with your doctor, because some of them can affect thyroid hormone test results
Before ordering this test it is worth asking your doctor whether it is needed at all in your case — according to the ATA, its clinical usefulness is limited

In Lithuania

Some laboratories in Lithuania offer the rT3 test among their specialist hormone tests. Before ordering it, it is worth discussing with a doctor what clinical benefit it would have in your particular case. For unexplained symptoms it is best to see a doctor, who will assess the whole clinical picture.

Full site in Lithuanian: proactiva.app

Frequently asked questions

›What is reverse T3 (rT3)?

It is a biologically inactive form of triiodothyronine that forms in the tissues from thyroxine (T4). Its structure is similar to active T3, but the iodine atoms are arranged differently, so the hormone does not act the way active T3 does.

›Is it worth testing reverse T3?

The American Thyroid Association states that in healthy, non-hospitalised people the rT3 test does not help establish hypothyroidism and is not clinically useful. Whether the test is meaningful in your case is best discussed with a doctor.

›How does reverse T3 differ from ordinary T3?

T3 is the active form of the hormone: it acts on cell receptors and regulates metabolism. Reverse T3 has the same chemical formula but a different arrangement of the iodine atoms, so it is biologically inactive and is broken down quickly in the body.

›Why is reverse T3 not a standard thyroid test?

According to the ATA, its level does not help tell whether a person has hypothyroidism, so routine assessment of thyroid function uses TSH, free T4 and, when needed, T3 — not rT3.

›Can the reverse T3 test be done in Lithuanian laboratories?

Some laboratories offer this test among their specialist hormone tests, but before ordering it, it is worth discussing with a doctor what clinical benefit it would have in your particular case.

›Is a low T3 with a normal TSH related to reverse T3?

Some people become interested in the rT3 test in that situation, but according to ATA data this marker does not help explain such a situation in healthy people — for unexplained symptoms it is best to see a doctor, who will assess the whole clinical picture.

Sources

Medicina practica — 2026-09-19

Related markers

This site is not an emergency service. If severe chest or head pain, breathlessness, confusion, difficulty speaking, heavy bleeding or another acute problem has come on suddenly — do not wait for tests, call 112.

This information is not a diagnosis and does not replace a consultation with a doctor. Reference ranges depend on the laboratory, your age and sex — always compare your result with the range printed on your own report.