FT3 (Free Triiodothyronine)

FT3 (Free Triiodothyronine) measures the unbound, biologically active fraction of triiodothyronine (T3) in the bloodstream. While total T3 reflects both bound and free hormone, free T3 is the metabolically active form that enters cells and exerts thyroid hormone effects. This test is typically requested alongside TSH and free T4 (fT4) to build a complete picture of thyroid function, particularly when hyperthyroidism is suspected, when total T3 results are confounded by binding protein abnormalities, or when monitoring treatment for thyroid disorders. A single blood sample is collected, usually in the morning, and analysed using a validated immunoassay platform in our UK laboratory.

What this check includes

• Free Triiodothyronine (fT3): The active thyroid hormone that regulates metabolism, heart rate, body temperature, and energy production. Unlike total T3, this measurement is unaffected by changes in binding proteins such as albumin or thyroid-binding globulin, making it a more reliable indicator of tissue-level thyroid hormone availability.

Understanding the results

• High fT3: May indicate hyperthyroidism (overactive thyroid), including Graves' disease, toxic nodular goitre, or thyroiditis. It can also occur in T3 toxicosis, where T3 is elevated while T4 remains normal.
• Low fT3: May be seen in hypothyroidism, but is more commonly a feature of non-thyroidal illness (sick euthyroid syndrome), severe stress, starvation, or certain medications. Low fT3 with normal TSH and fT4 can also occur in early or subclinical thyroid dysfunction.
• Normal fT3: Suggests that circulating active thyroid hormone is within the reference range, though results must always be interpreted alongside TSH, fT4, and clinical findings.

Why it matters

Free T3 is the hormone that directly drives your metabolic rate, influences cardiovascular function, and supports neurological and reproductive health. Measuring fT3 helps clinicians distinguish between different forms of thyroid disease, detect T3-predominant hyperthyroidism that might be missed by TSH and fT4 alone, and monitor response to antithyroid treatment. For patients experiencing symptoms such as unexplained weight loss, palpitations, heat intolerance, fatigue, or cognitive changes, fT3 provides essential diagnostic information that guides accurate diagnosis and effective management.