From the Guidelines
Triiodothyronine (T3) measures the active form of thyroid hormone in the bloodstream and is used to assess thyroid function, particularly in cases of suspected hyperthyroidism or when standard tests show normal results but thyroid dysfunction is still suspected. T3 testing is valuable for diagnosing hyperthyroidism, monitoring thyroid hormone replacement therapy, and evaluating unusual thyroid conditions like T3 toxicosis 1. In clinical practice, T3 testing helps identify cases where the body is not properly converting T4 to T3, which is essential since T3 is about four times more potent than T4 and directly affects metabolism, heart function, brain development, and bone maintenance. Some key points to consider when using T3 to assess thyroid function include:
- T3 testing is especially useful when symptoms are present, such as weight loss, heart palpitations, heat intolerance, and hyperactivity 1.
- The USPSTF recommends screening for thyroid dysfunction in asymptomatic populations, but notes that what constitutes an abnormal TSH level is uncertain 1.
- Accurate interpretation of serum TSH levels is further complicated by measurement variability and the sensitivity of TSH secretion to conditions other than thyroid dysfunction 1.
- Doctors may order both free T3 (the unbound, active portion) and total T3 tests depending on the specific diagnostic question, with free T3 providing a more accurate picture of the hormone actually available to tissues. When evaluating thyroid function, it is essential to consider the potential benefits and harms of screening and treatment, including the risk of labeling, false-positive results, and overdiagnosis and overtreatment 1.
From the Research
What is Triiodothyronine (T3)
- Triiodothyronine (T3) is a thyroid hormone produced by the thyroid gland, with the majority (80%) being produced extrathyroidally from thyroxine (T4) deiodination 2
- T3 is an important hormone for assessing thyroid function, particularly in cases of hyperthyroidism 3, 4
Clinical Uses of T3 Measurements
- The major clinical role for T3 measurements is the assessment of hyperthyroidism in patients with suppressed sensitive thyrotropin (sTSH) concentrations 3
- T3 measurements are also used to monitor thyroid hormone replacement therapy, evaluate amiodarone-induced thyrotoxicosis, and predict the outcome of antithyroid drug therapy in patients with Graves hyperthyroidism 3
- Free T3 (FT3) measurements are used to confirm and assess the level of hypo and hyperthyroidism, especially when the thyroidal status is unstable or when the hypothalamic-pituitary function is disturbed 2
Interpretation of T3 Results
- The free T3 index is clinically useful because serum total T3 may sometimes be misleading, and it can help diagnose T3-toxicosis in patients with a normal free T4 index and normal total T3 5
- T3 concentrations can remain within the reference interval in patients receiving TSH-suppressive levothyroxine (LT4) therapy, suggesting that formation of thyroid hormone metabolites is dependent on peripheral extrathyroidal metabolism of T4 6
Analytic Performance Goals for T3 Measurements
- Analytic recommendations for T3 measurements include a coefficient of variation (CV) < 5.2% for T3 and < 3.8% for FT3, with minimal cross-reactivity with other thyroid hormones 3