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
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).
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.
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.
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.