What It Is 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: Bound T3, the more abundant form, is attached to proteins that help transport the hormone through the body Free T3, the less abundant form, 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. Preparation 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, having your child wear a T-shirt or short-sleeved shirt can make things easier for the technician drawing the blood. The Procedure 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 A needle is inserted into a vein (usually in the arm inside of the elbow or on the back of the hand) and blood is withdrawn and collected in a vial or syringe.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. Getting the Results The blood sample will be processed by a machine. The results are commonly available after a few hours or the next day. Risks 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 kids 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. Back to Articles Related Articles Getting a Blood Test (Video) A blood test might sound scary, but it usually takes less than a minute. Watch what happens in this video for kids. Read More A to Z: Hashimoto's Thyroiditis Hashimoto's thyroiditis is a chronic (ongoing) condition where the body's immune system attacks the thyroid, a gland in the neck that makes thyroid hormone. Read More Blood Test (Video) These videos show what's involved in getting a blood test and what it's like to be the person taking the blood sample. Read More Blood Test: Thyroid Stimulating Hormone (TSH) Doctors may order TSH blood tests to diagnose and monitor treatment of a thyroid disorder or evaluate pituitary gland functioning. Read More Blood Test: T3 Total (Triiodothyronine) The T3 total test is part of an evaluation of thyroid function. It's particularly useful in diagnosing hyperthyroidism, which can cause symptoms such as a fast heart rate, weight loss, trembling and sweating. Read More Blood Test: Thyroid Peroxidase Antibodies The thyroid peroxidase antibodies test is primarily used to help diagnose and monitor autoimmune conditions involving the thyroid gland, including Hashimoto's thyroiditis and Graves disease. Read More Blood Test: Thyroglobulin Antibodies (TgAb) The thyroglobulin antibodies test is used to help diagnose autoimmune conditions involving the thyroid gland, or when thyroid disorders are suspected. Read More What Is the Thyroid? Do you know just how important the thyroid is? It helps you grow and affects your energy level. Read More Endocrine System The glands of the endocrine system and the hormones they release affect almost every cell, organ, and function of our bodies. Read More Endocrine System The endocrine system influences almost every cell, organ, and function of our bodies. It is instrumental in regulating mood, growth and development, metabolism, and sexual function, among other things. Read More Blood Test: T4 (Thyroxine) Doctors may order the T4 blood test if symptoms suggest any kind of thyroid disorder. Read More Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor. © 1995-2021 KidsHealth®. All rights reserved. Images provided by The Nemours Foundation, iStock, Getty Images, Veer, Shutterstock, and Clipart.com.