Blood Test: T3 Resin Uptake (T3RU)

What It Is

A T3 resin uptake (also called a T3 uptake or T3RU) is a blood test performed as part of an evaluation of thyroid function.

The thyroid is a gland in the neck that produces the hormones that help regulate many body processes, including growth, energy balance, body temperature, and heart rate.

Thyroid function involves the interaction of many hormones, including triiodothyronine (T3) and thyroxine (T4). Both of these hormones exist in two forms in the blood. The more abundant forms are bound to a carrier protein called thyroxin-binding globulin (TBG), which helps transport the hormones through the body. The less abundant forms circulate unattached or "free." Only the free forms of the thyroid hormones (free T4 and free T3) are available to affect body processes.

The T3 resin uptake is used by doctors to estimate the amount of TBG in the blood, and how much T4 and T3 in the blood is free form and available to affect the body.

If there's either too much or too little TBG in the blood, the measurements of total T3 and T4 levels will be affected, which can make it difficult for doctors to tell whether a person actually has a thyroid problem without also knowing the results of the T3 resin uptake.

Why It's Done

Doctors may order the T3 resin uptake when symptoms or previous blood tests seem to suggest thyroid dysfunction. When performed with other thyroid tests — such as those that check blood levels of T3, T4, and thyroid stimulating hormone (TSH) — this test can help diagnose hyperthyroidism (when the thyroid gland produces too much thyroid hormone) and hypothyroidism (when the thyroid gland isn't producing enough thyroid hormone).

Preparation

No special preparations are needed for a T3RU. However, certain medications, including seizure medications, steroids, and birth control pills, can affect the results, so it's important to tell the doctor about any 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. For an infant, the blood may be obtained by puncturing the heel with a small needle (lancet). If the blood is being drawn from a vein, the skin surface is cleaned with antiseptic and an elastic band (tourniquet) is placed 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 the blood for the test will only take a few minutes.

What to Expect

Either method (heel or vein withdrawal) of collecting a blood sample is only temporarily uncomfortable and can feel like a quick pinprick. Afterward, there may be some mild bruising, which should go away in a day or so.

Getting the Results

Results of the T3 resin uptake are commonly available in 1-2 days.

Risks

The T3RU is considered a safe procedure. However, as with many medical tests, some problems can occur with having blood drawn. These include:

• 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 any 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 for your child to look away when the needle is being inserted into the skin.

If You Have Questions

If you have questions about the T3 resin uptake test, speak with your doctor.