T4 (thyroxine) and T3 (triiodothyronine) are components of thyroid hormone, with T4 making up the largest amount. T4 is bound to proteins in the blood, with a small amount being unbound. This unbound T4 is called Free T4; this is the T4 available to be used by the body. Protein blood levels will affect the amount of Free T4. A measurement of free T4 is more accurate for diagnosing thyroid function, and is mainly used to diagnose how well the thyroid is functioning. Low T4 blood levels are seen in people who have had their thyroid surgically removed, who have an under-active pituitary gland or a non-functioning hypothalamous, kidney or liver disease, or cancer. High T4 blood levels are seen in people with infection of the thyroid gland, thyroid cancer, or Grave's disease. High levels are normally seen during hormone replacement therapy or during normal pregnancy and does not indicate disease. There are many medications that can affect Free T4 levels.
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