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 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.