Your thyroid is a gland located at the base of your neck. It produces hormones that are involved in controlling your metabolism. The hormones released by your thyroid interact with hormones released by other glands, especially the pituitary gland.
Your pituitary gland releases thyroid-stimulating hormones (TSH). TSH then stimulates your thyroid to release two hormones: triiodothyronine (T3) and thyroxine (T4). These hormones regulate your metabolism, growth, and body temperature.
Most of the T3 and T4 in your system bind to a protein called thyroxine-binding globulin (TBG). TBG’s primary function is to transport T3 and T4 in the bloodstream. The T3 and T4 that remains unattached is called “free.” When T3 and T4 are bound to proteins, they are inactive. When they are free in the bloodstream, they are active. T3 and T4 have an affect on almost every cell in the body. They control metabolism, help create protein, regulate bone growth and fat, and how the body uses energy.
The T3RU test helps your doctors estimate how much TBG is in your blood. A low T3RU value means there is a higher amount of TBG in the blood, while a high T3RU value means there is less TBG in your blood.
Your doctor will typically order a T3RU test if he or she suspects a problem with your thyroid. For example, your doctor might suspect you have hypothyroidism (an underactive thyroid) or hyperthyroidism (an overactive thyroid). He or she might also perform the test if you may have thyrotoxic periodic paralysis (temporary muscle weakness caused by very high levels of thyroid hormones).
Thyroid problems can cause many different symptoms, such as:
- sensitivity to cold
- weight gain
- high blood cholesterol
Hyperthyroidism, causes some opposite symptoms, including:
- sensitivity to heat
- more frequent bowel movements
- weight loss
- increased appetite
Other symptoms that could be related to thyroid problems include:
- difficulty sleeping
Because thyroid function is so complex, your doctor might also order tests for T3, T4, and/or TSH. Using results from more than one thyroid-related test can help your doctor fully understand any potential thyroid problems.
Pregnancy can decrease the levels of T3RU in your blood. Therefore it’s important to tell your doctor if you are pregnant so that he or she can interpret your results more accurately.
Several different kinds of drugs also can affect your T3RU test results. Make sure you tell your doctor about every medicine you are taking. This gives your doctor a chance to tell you if and when you should temporarily stop taking any of these medications.
Medications that can affect your T3RU test results include, but are not limited to:
- hormones (including some birth control pills)
- medications designed to treat thyroid issues
Your healthcare provider will draw a sample of your blood into a tube or vial. This sample will then be tested at a laboratory.
Typically, T3RU test results from 24 to 37 percent are considered normal.
Abnormal T3RU levels generally indicate thyroid problems such as hypothyroidism or hyperthyroidism. In other cases, however, abnormal results might indicate:
- kidney problems
- liver problems
- lack of protein in the diet
Pregnancy or estrogen use can also affect your T3RU results.