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.