FT4 (Free Thyroxine)

Free thyroxine (FT4) is the unbound, metabolically active fraction of thyroxine circulating in the bloodstream. Unlike total T4, which is influenced by binding proteins such as thyroxine-binding globulin, FT4 reflects the hormone immediately available to tissues. This test is used to investigate suspected thyroid dysfunction, to monitor treatment with levothyroxine or antithyroid drugs, and to distinguish between different patterns of thyroid disease when interpreted alongside thyroid-stimulating hormone (TSH). A single blood sample is required, typically collected into a serum tube. The analysis is usually performed using an immunoassay platform, most commonly chemiluminescent immunoassay (CLIA) or enzyme-linked immunosorbent assay (ELISA), depending on the laboratory's validated method.

What this check includes

• Free thyroxine (FT4): Measures the concentration of unbound thyroxine in serum. This is the biologically active form of the hormone that enters cells and influences metabolic rate, thermogenesis, cardiac function, and neurological development. A low FT4 with a high TSH suggests primary hypothyroidism, while a high FT4 with a low TSH suggests primary hyperthyroidism.

Understanding the results

FT4 results are interpreted in the context of the reference range provided by the laboratory and alongside TSH. In primary thyroid disease, FT4 and TSH typically move in opposite directions. In central (secondary) thyroid disease, both may be low or inappropriately normal. During pregnancy, in acute illness, or when certain medications are taken (for example, amiodarone, lithium, or high-dose biotin), FT4 results may require cautious interpretation. Always discuss results with a clinician who can integrate them with your symptoms, history, and other investigations.

Why it matters

Thyroid hormones regulate almost every organ system. Undiagnosed hypothyroidism can cause fatigue, weight gain, cold intolerance, constipation, and cognitive slowing, while untreated hyperthyroidism can lead to palpitations, weight loss, anxiety, and bone loss. Accurate measurement of FT4 supports timely diagnosis, safe monitoring of treatment, and appropriate dose adjustment, helping to prevent both under- and over-treatment.

• Free thyroxine (FT4)

No special preparation required.

For blood tests, we recommend drinking plenty of water beforehand to ensure smooth sample collection. If you are taking any regular medication, continue to do so unless advised otherwise by your doctor.