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Unit Code 9405:
T3 (Triiodothyronine), Reverse, Serum

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Useful For

rT3 determinations may be useful in the diagnosis of the "sick

euthyroid" syndrome.

Clinical Information

Reverse T3 (rT3) differs from 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.

rT3 is believed to be metabolically inactive.

 

The rT3 level tends to follow T4 level: low in hypothyroidism and high in

hyperthyroidism.

 

Increased levels of rT3 have been observed in starvation, anorexia

nervosa, severe trauma and hemorrhagic shock, hepatic dysfunction,

postoperative state, severe infection, and burn patients.  This appears to

be the result of conversion of T4 to rT3 being favored over the production

of T3.

Reference Values

0.18-0.51 nmol/L

(U for NIH)

11.7-33.2 ng/dL

Interpretation

Elevated rT3 values can be used to explain the low T3 values in

hospitalized or sick patients caused by the switchover in

deiodination functions.

Cautions

Generally, rT3 tests are not necessary since T3 should not be

ordered in hospitalized or sick patients.

 

This test should not be requested in patients who have recently

received radioisotopes, therapeutically or diagnostically, because

of potential assay interference. A recommended time period before

collection cannot be made because it will depend on the isotope

administered, the dose given and the clearance rate in the individual

patient. Specimens will be screened for radioactivity prior to analysis.

Radioactive samples received in the laboratory will be held and

assayed after the radioactivity has sufficiently decayed. This will

result in a test delay.

Clinical Reference

Moore WT, Eastman RC:  Diagnostic Endocrinology. Toronto,

BC Decker, 1990

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