T3 (Triiodothyronine), Free, Serum
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.
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
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.
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
Elevated free triiodothyronine (FT3) values are associated with thyrotoxicosis or excess thyroid hormone replacement.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Free triiodothyronine (FT3) is not a sensitive test for hypothyroidism.
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: 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. Ninth edition. WB Saunders Company, 1998