Thyroxine (T4) is the most common form of thyroid hormone that is released by the thyroid gland. Once it reaches the tissues, it is converted to another form of the thyroid hormone called triiodothyronine (T3), which is more active in regulating the body's chemical reactions.

In tests of thyroid function, a low T4 level may indicate the presence of low thyroid levels (hypothyroidism). Certain forms of liver or kidney disease may also cause a low T4 level, even though a thyroid problem may not be present.