Sorted By Test Name - Mayo Medical Laboratories

RT3
9405
T3
8613
(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
Clinical References: Moore WT, Eastman RC: Diagnostic Endocrinology. St, Louis, Mosby, 1990,
pp.182-183
T3 (Triiodothyronine), Total, Serum
Clinical Information: Thyroid hormones regulate a number of developmental, metabolic, and neural
activities throughout the body. The 2 main hormones secreted by the thyroid gland are thyroxine, which
contains 4 atoms of iodine (Thyroxine [T4]), and triiodothyronine (T3). T3 is also produced by conversion
(deiodination) of T4 in peripheral tissues. Circulating levels of T4 are much greater than T3 levels, but T3
is more metabolically active than T4, although its effect is briefer. Thyroid hormones circulate primarily
bound to carrier proteins (eg, thyroid-binding globulin [TBG], albumin); a small fraction circulates
unbound (free). Only the free forms are metabolically active. While both T3 and T4 are bound to TBG, T3
is bound less firmly than T4. Total T3 consists of both the bound and unbound fractions. In
hyperthyroidism both T4 and T3 levels are usually elevated, but in a small subset of hyperthyroid patients
only T3 is elevated (T3 toxicosis). In hypothyroidism T4 and T3 levels are decreased. T3 levels are
frequently low in sick or hospitalized euthyroid patients. See Thyroid Function Ordering Algorithm in
Special Instructions.
Useful For: Second-order testing for hyperthyroidism in patients with low thyroid stimulating hormone
values and normal T4 levels Diagnosis of T3 toxicosis
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