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Reverse Free Triiodothyronine (rT3), Serum/Plasma

Published: August 9, 2026Updated: August 17, 2026

A less commonly used thyroid hormone test that measures reverse free T3 in blood, usually to help evaluate thyroid hormone metabolism.

Quick summary

["This test is usually interpreted with other thyroid tests rather than by itself."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 17, 2026Editorial policy

Key facts at a glance

LOINC
3050-2
Short code
T3RU NFr SerPl
Synonyms
T3RU NFr SerPl
Category
thyroid
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
80
Ref high
200
Critical range
20 N/A – 400 N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
CPT
84436
SNOMED
302346007
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

What is this test?

Reverse triiodothyronine, also called rT3, is a form of the thyroid hormone T3 that has very little thyroid-hormone activity. This test measures the free, or unbound, amount of reverse T3 in serum or plasma.

Why might I need this test?

This test may be ordered when a clinician needs more information about how the body is converting thyroid hormones. It is generally used alongside tests such as thyroid-stimulating hormone (TSH) and free T4 rather than as a stand-alone thyroid screening test.

How is the test performed?

A blood sample is drawn from a vein and analyzed in a laboratory. The sample is tested to determine the concentration of free reverse T3 in the serum or plasma portion of the blood.

How do I prepare for the test?

Special preparation is usually not required. Tell your clinician about thyroid medications, other medicines or supplements you take, and any recent illness, because these can affect thyroid test results.

Reference range

Adult: 80-200 ng/dLN/A

Specimen: serum

Take your thyroid medication after the blood draw if applicable. No fasting required.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Reverse free T3

Frequently asked questions

Is reverse free T3 the same as a routine free T3 test?
No. Free T3 measures the active form of T3, while reverse free T3 measures a much less active form. Reverse free T3 is not commonly used as a routine screening test.
Do I need to stop taking thyroid medication before this test?
Do not stop prescribed medication unless your clinician instructs you to do so. Thyroid medicines can affect the result, so tell the laboratory and your clinician about everything you are taking.
Can a normal result rule out a thyroid disorder?
No. A normal reverse free T3 result does not exclude thyroid disease. Your clinician will interpret it together with thyroid symptoms, medical history, and other laboratory tests.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 17, 2026. · Read our editorial policy