T3 (Triiodothyronine), Free and Total, Serum
Triiodothyronine (T3) is a second-order test in follow-up to low thyroid stimulating hormone values in the evaluation of patients suspected of having hyperthyroidism caused by excess T3 (T3 toxicosis).
Free T3 (FT3) levels can be helpful in evaluating patients with altered levels of binding proteins, such as pregnant patients, patients receiving estrogens and anabolic steroids, and patients with dysalbuminemia. Some investigators recommend FT3 for monitoring thyroid replacement therapy, although its clinical role is not precisely defined. FT3 also provides a further confirmatory test for hyperthyroidism to supplement the thyroxine (T4), sensitive thyrotropin, and total T3 assays.
Thyroid hormones regulate a number of developmental, metabolic, and neural activities throughout the body. The 2 main hormones secreted by the thyroid gland, under the influence of thyroid stimulating hormone, are thyroxine (T4), which contains 4 atoms of iodine, and triiodothyronine (T3). T3 is also produced by conversion (deiodination) of T4 in peripheral tissues; 20% of T3 occurs from direct synthesis and 80% occurs from conversion of T4 to T3. Circulating levels of T3 are much lower than T4 levels, but T3 is more metabolically active than T4, although its effect is briefer.
In circulation, 99.7% of T3 is bound to proteins, primarily thyroxine-binding globulin. Free T3 is the metabolically active form of T3 and accounts for 0.3% of the total T3.
In hyperthyroidism, both T4 and T3 levels are usually elevated, but in a small subset of hyperthyroid patients (T3 toxicosis) only T3 is elevated.
In hypothyroidism, T4 and T3 levels are decreased. T3 levels are also often low in sick or hospitalized euthyroid patients.
> or =1 year: 2.0-3.5 pg/mL
> 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.
T3 often is low in sick or hospitalized patients, so it is not a good indicator of hypothyroidism.
Abnormal levels of binding proteins to T3 (primarily albumin and thyroxine-binding globulin) may cause either high or low T3 concentrations in euthyroid patients.
Elevations in free T3 are associated with thyrotoxicosis or excess thyroid replacement therapy.
Triiodothyronine (T3) is not a reliable marker for hypothyroidism and is not useful for general screening of the population without a clinical suspicion of hyperthyroidism.
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