T3 (Triiodothyronine), Total, Serum
Second-order testing for hyperthyroidism in patients with low thyroid stimulating hormone values and normal T4 levels
Diagnosis of T3 toxicosis
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.
> or =1 year: 80-190 ng/dL
Triiodothyronine (T3) values >180 ng/dL in adults or >200 ng/dL in children are consistent with hyperthyroidism or increased thyroid hormone-binding proteins.
Abnormal levels (high or low) of thyroid hormone-binding proteins (primarily albumin and thyroid-binding globulin) may cause abnormal T3 concentrations in euthyroid patients.
Triiodothyronine (T3) is not a reliable marker for hypothyroidism.
T3 is not useful for general screening of the population without a clinical suspicion of hyperthyroidism.
Some patients who have been exposed to animal antigens, either in the environment or as part of treatment or imaging procedures, may have circulating antianimal antibodies present. These antibodies may interfere with the assay reagents to produce unreliable results.
Clinical Reference Provides recommendations for further in-depth reading of a clinical nature
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