Thyroid Biomarker
Collection Type: Blood
Related System: Thyroid
Free T3 (triiodothyronine, FT3) is the unbound, biologically active form of the thyroid hormone T3 circulating in blood. The FT3 test measures the free T3 available to tissues (not the protein-bound fraction) and helps assess thyroid function and metabolic state. It is used to detect or monitor hyperthyroidism (e.g., Graves’ disease, toxic nodules, T3 toxicosis) and to evaluate suspected hypothyroidism or central (pituitary) dysfunction. Symptoms prompting testing include unexplained weight change, palpitations, heat/cold intolerance, fatigue, hair loss, or mood changes. FT3 can vary with age (often lower in older adults), pregnancy, acute illness, medications and sex hormones.
Run our symptom checker to see if this test is right for you
Symptom Checker2.3-4.2 pg/mL OR 3.5-6.8 pmol/L
Q: What is triiodothyronine T3 free?
A: Free triiodothyronine (free T3) is the unbound, biologically active form of the thyroid hormone triiodothyronine circulating in blood. It regulates metabolism, heart rate, temperature, and growth. Most T3 is protein-bound; free T3 reflects hormone available to tissues and is measured to evaluate thyroid function. Abnormal free T3 levels suggest hyperthyroidism, hypothyroidism, or altered peripheral conversion, and can be affected by illness or medications.
Q: What is the normal range for free T3?
A: The usual reference range for free T3 (FT3) is about 3.1–6.8 pmol/L (approximately 2.0–4.4 pg/mL). Individual laboratory ranges and assay methods vary, so values slightly outside this window aren’t always diagnostic. Discuss your FT3 together with TSH, free T4, symptoms and clinical context with your clinician for proper interpretation.
Q: How do I fix low free T3?
A: First, confirm low free T3 with repeat testing and evaluate causes (hypothyroidism, illness, medications). Treat underlying disease and optimize thyroid-hormone therapy—sometimes adjusting levothyroxine or adding liothyronine (T3) under specialist supervision. Check for and correct iron, selenium or iodine deficiencies, review interacting drugs, manage chronic illness/inflammation, and improve sleep, stress and nutrition. See an endocrinologist for monitoring.
Q: Can low free T3 cause weight gain?
A: Yes. Free T3 is the active thyroid hormone that drives metabolism; low free T3 (seen in hypothyroidism or non-thyroidal illness) reduces basal metabolic rate, lowers energy expenditure, can increase fatigue and fluid retention, and may lead to weight gain or make weight loss difficult. Treating the underlying thyroid condition and normalizing thyroid hormones often improves weight, but evaluation and management should be done with a clinician.
Q: What are the symptoms of low T3?
A: Low T3 may cause fatigue, persistent weight gain or difficulty losing weight, increased sensitivity to cold, dry skin, hair thinning or loss, constipation, muscle weakness and cramps, slowed thinking or poor concentration, low mood or depression, slowed heart rate, menstrual irregularities, hoarseness and facial puffiness. Symptoms vary by severity and may overlap with other conditions see a clinician for testing.
Q: What is the function of the T3 hormone?
A: Triiodothyronine (T3) is the active thyroid hormone that regulates basal metabolic rate and energy use. It increases oxygen consumption and heat production, stimulates carbohydrate, fat and protein metabolism, and influences heart rate and contractility. T3 promotes growth, brain development and bone maturation, and acts by entering cells and altering gene transcription to control cellular metabolism and overall physiological homeostasis.