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.
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Thyroid hormones regulate a number of developmental, metabolic, and neural activities throughout the body. The thyroid gland synthesizes 2 hormones: thyroxine (T4), and triiodothyronine (T3). T3 production in the thyroid gland constitutes approximately 20% of the total T3; the rest is generated by the conversion (deiodination) of T4 to T3 in peripheral tissues. Circulating levels of T4 are much greater than T3 levels but T3 is biologically the most active hormone (3 to 4 times more potent than T4), although its effect is briefer due to its shorter half-life compared to T4.
Thyroid hormones circulate primarily bound to carrier proteins (eg, thyroid-binding globulin [TBG], prealbumin and albumin); whereas only a small fraction circulates unbound (free). The free form of T3 is the biologically active fraction. 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 also often low in sick or hospitalized euthyroid patients.
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.
> or =1 year: 2.0-3.5 pg/mL
Adult (> =20 years): 80-200 ng/dL
0-5 days: 73-288 ng/dL
6 days-2 months: 80-275 ng/dL
3 months-11 months: 86-265 ng/dL
1 year-5 years: 92-248 ng/dL
6 years-10 years: 93-231 ng/dL
11 years-19 years: 91-218 ng/dL
Triiodothyronine (T3) values >200 ng/dL in adults or >age related cutoffs in children are consistent with hyperthyroidism or increased thyroid hormone binding proteins.
Abnormal levels of binding proteins to T3 (primarily albumin and thyroxine-binding globulin) may cause abnormal T3 concentrations in euthyroid patients.
Elevations in free T3 are associated with thyrotoxicosis or excess thyroid replacement therapy.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
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.
Therapy with amiodarone can lead to depressed T3 values.
Phenytoin, phenylbutazone, and salicylates cause release of T3 from the binding proteins, thus leading to a reduction in the total T3 hormone level at normal free T3 levels.
Autoantibodies to thyroid hormones can interfere with the assay.
Binding protein anomalies may cause values which deviate from the expected results. Pathological concentrations of binding proteins can lead to results outside the reference range, although the patient may be in a euthyroid state. Free T3 or free T4 testing is indicated in these cases.
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.
In patients receiving therapy with high biotin doses (ie, >5 mg/day), no sample should be taken until at least 8 hours after the last biotin administration.
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