A T3 test is performed as part of an evaluation of thyroid
function. It measures the blood level of the hormone T3
(triiodothyronine), some of which is produced directly by the
thyroid gland. Most T3 in the blood, however, is produced
elsewhere in the body where T4 (the major thyroid hormone
produced by the thyroid gland) is chemically converted to T3. T3
helps control many body functions including growth, body
temperature, and heart rate. T3 exists in two forms in the
blood: the more abundant form, which is attached to proteins
that help transport the hormone through the body the less
abundant form, called free T3, which circulates unattached The
T3 total test, the most common type of T3 blood test, measures
both the bound and free forms of T3 in your child's blood. Why
It's Done Doctors may order the T3 test when a child's symptoms
or previous blood tests suggest thyroid dysfunction. The T3 test
is particularly useful in diagnosing hyperthyroidism, a
condition in which the thyroid overproduces hormones, cau