Triiodothyronine
(T3)

Alternate Names

  • Total T3 and Free T3
  • T3 Radioimmunoassay

T3 (triiodothyronine) and T4 (thyroxine) are components of thyroid hormone, with T3 only making up 7 to 10 percent of the total amount. T3 is mostly bound to proteins in the blood, with a small amount being unbound, called Free T3, which is available for use by the body. Protein blood levels will affect the amount of T3 that is both bound and free. The measurement of free T3 is more accurate for diagnosing thyroid function. This test measures both Free T3 and Total T3 levels, and is mainly used to diagnose how well the thyroid is functioning.

Low T3 blood levels are seen in people who have had their thyroid gland surgically removed, or who an under-active pituitary gland or non-functioning hypothalamous. Levels are also low with kidney or liver disease or cancer, poor nutrition, and low iodine levels. High T3 blood levels are seen in people with infection of the thyroid gland, thyroid cancer, Grave's disease, hepatitis, or inherited diseases that result in low blood protein levels. High T3 levels are also seen during normal pregnancy, which does not indicate thyroid disease. There are many medications that can affect Free T3 levels.

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