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Test ID: RT3    
T3 (Triiodothyronine), Reverse, Serum

Useful For Suggests clinical disorders or settings where the test may be helpful

Aids in the diagnosis of the "sick euthyroid" syndrome

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

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.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

10-24 ng/dL

Interpretation Provides information to assist in interpretation of the test results

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.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

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

Clinical Reference Provides recommendations for further in-depth reading of a clinical nature

Moore WT, Eastman RC: Diagnostic Endocrinology. St. Louis, Mosby, 1990, pp.182-183