Mayo Test Catalog, (Sorted By Test Name) - Mayo Medical ...

FRT3
9404
RT3
9405
T3, TOTAL
> or =1 year: 80-190 ng/dL
Clinical References: 1. Hay ID, Klee GG: Linking medical needs and performance goals: clinical
and laboratory perspectives on thyroid disease. Clin Chem 1993;39:1519-1524 2. Klee GG: Clinical
usage recommendations and analytic performance goals for total and free triiodothyronine
measurements. Clin Chem 1996;42:155-159
T3 (Triiodothyronine), Free, Serum
Clinical Information: Normally triiodothyronine (T3) circulates tightly bound to thyroxine-binding
globulin and albumin. Only 0.3% of the total T3 is unbound (free); but the free fraction is the active
form. In hyperthyroidism, both thyroxine (tetraiodothyronine; thyroxine [T4]) and T3 levels (total and
free) are usually elevated, but in a small subset of hyperthyroid patients (T3 toxicosis) only T3 is
elevated. Generally, free T3 (FT3) measurement is not necessary since total T3 will suffice. However,
free T3 levels may be required to evaluate clinically euthyroid patients who have an altered distribution
of binding proteins (eg, pregnancy, dysalbuminemia). Some investigators recommend the free T3 assay
for monitoring thyroid replacement therapy, although its clinical role is not precisely defined.
Useful For: Free triiodothyronine (T3) is a second- or third-level test of thyroid function. It provides
further confirmation of hyperthyroidism, supplementing the tetraiodothyronine (T4), sensitive
thyrotropin (sTSH), and total T3 assays. Evaluating clinically euthyroid patients who have an altered
distribution of binding proteins. Monitoring thyroid hormone replacement therapy.
Interpretation: Elevated free triiodothyronine (FT3) values are associated with thyrotoxicosis or
excess thyroid hormone replacement.
Reference Values:
> or =1 year: 2.0-3.5 pg/mL
Clinical References: 1. Hay ID, Klee GG: Biochemical testing of thyroid function. Endocrinol
Metab Clin North Am 1997;26(4):763-775 2. Wilson JD, Foster DW, Kronenburg MD, Larsen PR:
Williams Textbook of Endocrinology. 9th edition. WB Saunders Company, 1998
T3 (Triiodothyronine), Reverse, Serum
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.
Useful For: Aids in the diagnosis of the "sick euthyroid" syndrome
Interpretation: 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
Reference Values:
10-24 ng/dL
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