Total T4 is Bound and Free

Thyroxine (T4) is a major thyroid hormone that contains four molecules of iodine. It exists in both bound and free forms in the blood and the total T4 value reflects both. While most T4 is bound to protein for transport through the blood, a small amount occurs as free T4 which is more readily available for use by cells. 

However, T4 must be converted to T3 to exert most of its actions. Reduced thyroid hormone activity is associated with slower metabolism, decreased energy expenditure, weight gain, insulin resistance, high blood pressure, and compromised cardiovascular health.

 A low T4 is associated with hypothyroidism, iodine insufficiency, decreased pituitary function, hypothalamic failure, cirrhosis, renal failure, Cushing syndrome, cognitive dysfunction, and the use of certain medications including anabolic steroids, anti-thyroid drugs, and lithium.

An elevated T4 is associated with hyperthyroidism, acute thyroid inflammation, elevated thyroid binding globulin, hepatitis, pregnancy, and the use of certain medications.

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