FT4 (Free Thyroxine)

Free thyroxine (FT4) is the unbound, biologically active portion of thyroxine circulating in your bloodstream. Unlike total thyroxine, which is largely carried by transport proteins, FT4 is the fraction available to enter cells and influence metabolism, heart rate, body temperature, and energy production. This test is for anyone experiencing symptoms of an overactive or underactive thyroid, such as unexplained weight change, fatigue, palpitations, heat or cold intolerance, or mood changes. It is also used to monitor thyroid function in people already receiving treatment for thyroid disorders and to investigate suspected pituitary or hypothalamic conditions.

Free thyroxine
- Free thyroxine (FT4): measures the concentration of unbound thyroxine in the blood, providing a direct indication of the amount of active hormone available to tissues.

Why it matters
FT4 gives a clear picture of how much active thyroid hormone is available to your body. Because it is not affected by changes in binding proteins, it is a more reliable indicator of thyroid status than total thyroxine in many clinical situations. Measuring FT4 alongside TSH helps distinguish between primary thyroid disorders and central (pituitary or hypothalamic) causes, guiding accurate diagnosis and effective treatment. Whether you are investigating new symptoms or monitoring an existing thyroid condition, FT4 provides essential information for your clinician to interpret and act upon.