Triiodothyronine (T3) is the active form of thyroid hormone and is found in bound and free form in circulation with the free form being the most biologically active and available. T3 contains three atoms of iodine and can be formed from T4 (4 atoms of iodine) in several tissues in the body. This conversion relies on the trace mineral selenium but can also be influenced by liver and kidney function, nutrition status, heavy metal exposure, smoking, and alcohol intake.
A low total T3 can be associated with hypothyroidism, euthyroid sick syndrome, iodine insufficiency, malnutrition, protein depletion, liver disease, dysfunction of the pituitary or hypothalamus, acute stroke, and the use of certain medications including anabolic steroids and high-dose salicylates.
Elevated total T3 is seen with hyperthyroidism, acute thyroid inflammation, hepatitis, pregnancy, and the use of certain medications including estrogen, oral contraceptives, and methadone.
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