Thyroxine, Total
(T4)

Alternate Names

  • Thyroxine Screen

T4 (thyroxine) is a component of thyroid hormone; making up 90 percent of that hormone. 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 that is available to be used by the body. A measurement of Total T4 is an indication of how well the thyroid is functioning, is also used to monitor the effectiveness of thyroid replacement therapy, and as a screen for the presence of an under-active thyroid gland in newborn babies. Low Total T4 blood levels are seen in people who have had their thyroid surgically removed, who have low blood protein levels, kidney or liver disease, low amounts of iodine in the diet, a non-functioning hypothalamus, or an under-active pituitary gland. High Total T4 blood levels are seen in people with hepatitis, high blood protein levels, thyroid infection or cancer, Grave's disease, or during normal pregnancy. There are many medications that can affect Total T4 levels.

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