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EPIC CODE: LAB138 T3 (Triiodothyronine), Reverse, Serum

Additional Codes

Sunquest:   T3REVM
Mayo:          RT3
Previously:  ARUP 2007918

Reporting Name

T3 (Triiodothyronine), Reverse, S

Useful For

Aiding in the diagnosis of the sick euthyroid syndrome

Performing Laboratory

Mayo Clinic Laboratories in Rochester

Specimen Type


Specimen Required

Collection Container/Tube:

Preferred: Red top

Acceptable: Serum gel

Submission Container/Tube: Plastic vial

Specimen Volume: 0.8 mL

Specimen Minimum Volume

0.4 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Serum Refrigerated (preferred) 28 days
  Ambient  28 days
  Frozen  28 days

Reference Values

10-24 ng/dL

Day(s) Performed

Monday through Friday

Test Classification

This test was developed, and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the US Food and Drug Administration.

CPT Code Information


LOINC Code Information

Test ID Test Order Name Order LOINC Value
RT3 T3 (Triiodothyronine), Reverse, S 3052-8


Result ID Test Result Name Result LOINC Value
9405 T3(Triiodothyronine),Reverse,S 3052-8

Clinical Information

Reverse triiodothyronine (rT3) differs from triiodothyronine (T3) in the positions of the iodine atoms attached to the aromatic rings. The majority of rT3 found in the circulation is formed by peripheral deiodination (removal of an iodine atom) of T4 (thyroxine). rT3 is believed to be metabolically inactive.


The rT3 level tends to follow the T4 level: low in hypothyroidism and high in hyperthyroidism. Additionally, increased levels of rT3 have been observed in starvation, anorexia nervosa, severe trauma and hemorrhagic shock, hepatic dysfunction, postoperative states, severe infection, and in burn patients (ie, sick euthyroid syndrome). This appears to be the result of a switchover in deiodination functions with the conversion of T4 to rT3 being favored over the production of T3.


In hospitalized or sick patients with low triiodothyronine (T3) values, elevated reverse triiodothyronine (rT3) values are consistent with sick euthyroid syndrome. Also, the finding on an elevated rT3 level in a critically ill patient helps exclude a diagnosis of hypothyroidism.


The rT3 is high in patients on medications such as propylthiouracil, ipodate, propranolol, amiodarone, dexamethasone, and the anesthetic agent halothane. Dilantin decreases rT3 due to the displacement from thyroxine-binding globulin, which causes increased rT3 clearance.


To convert from ng/dL to nmol/L, multiply the ng/dL result by 0.01536.


Generally, reverse triiodothyronine tests are not necessary since triiodothyronine should not be ordered in hospitalized or sick patients.

Clinical Reference

1. Bowerbank SL, Carlin MG, Dean JR: A direct comparison of liquid chromatography-mass spectrometry with clinical routine testing immunoassay methods for the detection and quantification of thyroid hormones in blood serum. Anal Bioanal Chem. 2019 May;411(13):2839-2853

2. Moore WT, Eastman RC: Diagnostic Endocrinology. Mosby; 1990:182-183

Method Description

Reverse triiodothyronine-13C (rT3-13C) is added to patient sample as an internal standard. rT3 and rT3-13C are dissociated with a protein precipitation and then extracted from the specimens using a solid-phase cartridge. The eluate is dried down under nitrogen, reconstituted with mobile phase, and analyzed by liquid chromatography-tandem mass spectrometry using multiple reactions monitoring in the positive mode.(Zhang Y, Conrad AH, Conrad GW: Detection and quantification of 3, 5, 3'-triiodothyronine and 3, 3', 5'-triiodothyronine by electrospray ionization tandem mass spectrometry. J Am Soc Mass Spectrom. 2005 Nov;16[11]:1781-1786; Sakai H, Nagao H, Sakurai M, et al. Correlation between serum levels of 3,3',5'-triiodothyronine and thyroid hormones measured by liquid chromatography-tandem mass spectrometry and immunoassay PLoS One. 2015;10[10]:e0138864. doi: 10.1371/journal.pone.0138864. Erratum in: PLoS One. 2016;11[7]:e0159169)

Report Available

2 to 6 days

Specimen Retention Time

14 days

Reject Due To

Gross hemolysis Reject
Gross lipemia OK
Gross icterus OK

NY State Approved


Method Name

Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)

Portions of this test are covered by patents held by Quest Diagnostics.


If not ordering electronically, complete, print, and send a General Request (T239) with the specimen. 

Secondary ID

UnityPoint Health Peoria Additional Information: