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, causing symptoms such as a fast heart rate, weight loss, trembling and sweating.
No special preparations are needed for this test. However, certain medications, including seizure medication, steroids, and birth control pills, can affect the results, so it's important to tell the doctor about any medication your child is taking.
On the day of the test, it may help to have your child wear a short-sleeve shirt to allow easier access for the technician who will be drawing the blood.
A health professional will usually draw the blood from a vein, after cleaning the skin surface with antiseptic, and placing an elastic band (tourniquet) around the upper arm to apply pressure and cause the veins to swell with blood. A needle is inserted into a vein (usually in the arm inside of the elbow or on the back of the hand) and blood is withdrawn and collected in a vial or syringe.
After the procedure, the elastic band is removed. Once the blood has been collected, the needle is removed and the area is covered with cotton or a bandage to stop the bleeding. Collecting blood for this test will only take a few minutes.
What to Expect
Either method (heel or vein withdrawal) of collecting a sample of blood is only temporarily uncomfortable and can feel like a quick pinprick. Afterward, there may be some mild bruising, which should go away in a few days.
The blood sample will be processed by a machine. The results are commonly available after a few hours or the next day.
The T3 test is considered a safe procedure. However, as with many medical tests, some problems can occur with having blood drawn:
fainting or feeling lightheaded
hematoma (blood accumulating under the skin causing a lump or bruise)
pain associated with multiple punctures to locate a vein
Helping Your Child
Having a blood test is relatively painless. Still, many children are afraid of needles. Explaining the test in terms your child can understand might help ease some of the fear.
Allow your child to ask the technician any questions he or she might have. Tell your child to try to relax and stay still during the procedure, as tensing muscles and moving can make it harder and more painful to draw blood. It also may help if your child looks away when the needle is being inserted into the skin.
If You Have Questions
If you have questions about the T3 test, speak with your doctor.