Reverse Triiodothyronine (rT3) Optimal Testing Recommendations
Not useful for routine thyroid function testing.
Guidelines for Test Utilization
What does the test tell me?
Reverse triiodothyronine (rT3) is the metabolically inactive iodothyronine produced by the deiodination of the prohormone thyroxine (T4).
rT3 has limited applications. The concentration is increased in sick euthyroid syndrome and by administering medications, such as amiodarone which inhibit the metabolism of rT3. [back to top]
When should I order this test?
May be helpful in the investigation of rare genetic syndromes, such as resistance to thyroid hormone due to defective thyroid receptor α and monocarboxylate transporter 8 (MCT8) deficiency. [back to top]
When should I NOT order this test?
rT3 should not be used for thyroid function evaluation.
Generally, it is unnecessary to use rT3 to diagnose sick euthyroid syndrome since thyroid hormones should not be ordered in hospitalized or ill patients unless thyroid function disorder is the suspected cause of hospital admission.
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How should I interpret the result?
rT3 can be elevated when the body is undergoing severe stress. It is not clear whether the results point to any specific treatment plan. [back to top]
Is the test result diagnostic/confirmatory of the condition? If not, is there a diagnostic/confirmatory test?
Not a diagnostic/ confirmatory test. In contrast, thyroid-stimulating hormone (TSH) is recommended as the main screening test for thyroid dysfunction in most clinical circumstances. [back to top]
Are there factors that can affect the lab result?
Cross-reaction with other thyroid hormones and/or heterophile antibodies in immunoassays could result in a false increase in rT3. [back to top]
Are there considerations for special populations?
Concurrent illness affects the rT3 concentration. [back to top]
What other tests might be indicated?
The most commonly measured thyroid hormones for evaluating thyroid function are TSH and free T4. [back to top]
Halsall DJ, Oddy S. Clinical and laboratory aspects of 3,3′,5′-triiodothyronine (reverse T3). Annals of Clinical Biochemistry. 2021;58(1):29-37. doi:10.1177/0004563220969150
Iwayama H, Kakita H, Iwasa M, Adachi S, Takano K, Kikuchi M, Fujisawa Y, Osaka H, Yamada Y, Okumura A, Hirani K, Weiss RE, Refetoff S. Measurement of Reverse Triiodothyronine Level and the Triiodothyronine to Reverse Triiodothyronine Ratio in Dried Blood Spot Samples at Birth May Facilitate Early Detection of Monocarboxylate Transporter 8 Deficiency. Thyroid. 2021 Sep;31(9):1316-1321. DOI: 10.1089/thy.2020.0696. Epub 2021 Jul 2. PMID: 34049438; PMCID: PMC8558056.
Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, 6th Edition