Total triiodothyronine (T3) measures the combined concentration of both protein-bound and free T3 in the serum, representing the predominant biologically active thyroid hormone. Approximately 80% of circulating T3 is derived from peripheral deiodination of thyroxine (T4), with the remainder secreted directly by the thyroid gland. Total T3 is regulated by the hypothalamic-pituitary-thyroid axis via TSH and is a key indicator of thyroid hormone action at the tissue level. T3 exerts its effects by binding to nuclear thyroid hormone receptors, regulating gene transcription involved in metabolism, thermogenesis, cardiac function, and neurodevelopment. It has approximately three to four times greater biological potency than T4 due to higher receptor affinity and faster cellular uptake. Circulating T3 is predominantly bound to thyroxine-binding globulin (TBG), transthyretin, and albumin, with only about 0.3% existing in the free, biologically active form.
Total T3 measures the level of triiodothyronine, one of the main hormones made by your thyroid gland, in your blood. This hormone helps control your metabolism, energy levels, heart rate, and many other body functions. When T3 is too high, it may suggest your thyroid is overactive; when it is too low, it can indicate an underactive thyroid or that your body is under significant stress from illness or poor nutrition. This test is usually interpreted alongside other thyroid tests, especially TSH, to give your doctor a complete picture of how your thyroid is functioning. Abnormal results do not always mean a thyroid disease is present, as many factors including medications and other illnesses can affect T3 levels.
When elevated: Elevated total T3 is most commonly associated with hyperthyroidism, including Graves' disease, toxic multinodular goiter, and toxic adenoma. Isolated T3 elevation with normal T4 (T3 toxicosis) may represent early or mild hyperthyroidism. Elevated TBG levels due to estrogen excess, pregnancy, or oral contraceptive use can raise total T3 without true hyperthyroid physiology. When low: Low total T3 may reflect primary or central hypothyroidism, but is also commonly seen in non-thyroidal illness syndrome (euthyroid sick syndrome), where peripheral conversion of T4 to T3 is impaired. Malnutrition, prolonged fasting, and severe systemic illness can suppress T3 without intrinsic thyroid pathology. Certain medications, including glucocorticoids, amiodarone, and propylthiouracil, reduce T3 by inhibiting peripheral deiodination.
Total T3 is relevant to athletes because it directly influences metabolic rate, energy expenditure, and thermogenesis—all critical for performance and recovery. Elevated T3 increases caloric demand and can accelerate fatigue if training load isn't matched by nutrition; conversely, suppressed T3 from overtraining or underfueling may impair recovery and reduce power output. Monitoring T3 alongside TSH and Free T4 helps identify whether fatigue or performance plateaus stem from thyroid dysfunction or overtraining syndrome.
Turnaround Time
4 days (up to 14 days)
Fasting Required
No
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