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Also known as:  T3 Resin Uptake, T3RU, Thyroxine-binding ratio.

What is the T3-Uptake test?

T3-Uptake test, unlike its name, is used to determine the binding capacity of TBG. TBG or thyroxine-binding globulin is a protein that binds to T3 and T4, converting them into their bound form and limiting their activity. T3 and T4 in their free form are active and serve their functions. T3, also known as triiodothyronine, and T4, also known as tetraiodothyronine and thyroxine, are the two hormones produced by the thyroid gland.
The thyroid gland is a butterfly-shaped endocrine gland present at the base of your neck, which regulates many processes of your body like growth, metabolism, body temperature, balances the energy and heart rate with the help of these two hormones. The thyroid gland functions are influenced by the amount of iodine present in your diet. The thyroid gland cells absorb iodine from the blood and incorporate it into the thyroid hormones.

T3 is produced in its active form, but T4 is produced as a prohormone and is later converted into active T3 by enzymes in the kidney or liver. The thyroid gland also has separate c-cells to produce calcitonin, which is another hormone playing a role in your blood and bone, calcium and phosphate levels.

What is this test used for?

T3-Uptake test tells us the amount of TBG in your blood and levels of T3 and T4, which are free and effective to your body's functions. If the levels of T3-uptake are high, that means excess levels of T4 and T3 are bound to TBG, and if the T3-uptake levels are low, that means excess free T3 and T4 levels are present in the blood. Abnormal TBG levels indirectly indicate it affected T3 and T4 levels, which makes this test crucial for diagnosing thyroid diseases.

Why and when do you need a T3-Uptake test?

T3-Uptake test is ordered if your doctor has suspected any thyroid abnormality. Suppose the T3-Uptake test is paired with T3, T4, and TSH blood levels test. In that case, it helps diagnose the most common thyroid disease, hyperthyroidism (excess production of thyroid hormones by the thyroid gland) and hypothyroidism (insufficient production of thyroid hormones by the thyroid gland). Thyroid abnormalities can be diagnosed by common symptoms like:

  • Anxiety
  • Irregular menstrual cycle
  • General weakness
  • Dry skin
  • Abnormal heart rate
  • Sudden weight fluctuations
  • Excess hair fall

Your doctor can also order a T3-Uptake test to follow up on your past thyroid diseases.

What kind of sample is required for the T3-Uptake test?

For the T3-Uptake test, a sample of blood is required. The procedure of the test will be like any other blood test. First, an antiseptic will be rubbed at the point of puncture and then blood will be drawn from the vein of your arm or the back of your hand using a needle and will be collected in a container attached to the needle. The blood will then be checked in the laboratory for your TBG levels.

Do you need to prepare for the test?

No special preparation or fasting is needed before taking the T3-Uptake test. Still, certain medicines and drugs should not be taken before this test to avoid unnecessary alterations in the blood TBG levels. The medicines which alter T3-Uptake levels are:

  • Steroids (such as birth control pills, including androgens and estrogen).
  • Heparin.
  • Warfarin.
  • Thyroid drugs.
  • Certain antipsychotic drugs.

Are there any risks to this test?

Because it is a simple blood test, it does not associate any serious risks, but you may feel discomfort by the needle's prick; you may also get a little bruising later. Rare serious complications of the blood test include:

  • Excessive bleeding.
  • Hematoma (blood accumulation under the skin).
  • Skin infection at the puncture point.
  • Fainting.

If you experience any of these conditions after the test, you should immediately talk to your doctor.

What does the test result mean?

The normal range of T3-Uptake in adults is 25%-35%. Keep in mind that T3-Uptake results alone are not enough to determine the proper functioning of the thyroid. Your doctor should always check the results and come to a conclusion, but below are listed a few conditions that alter the T3-Uptake levels of the blood.

Low T3-Uptake levels:

Decreased T3-Uptake levels indicate higher TBG levels in the blood. TBG may be increased due to:

  • Pregnancy.
  • Thyroid tumor.
  • Liver diseases.
  • Certain steroids.
  • High level of estrogen in the blood.

T3-Uptake may also be decreased if the thyroid gland produces less than normal quantities of T3 and T4; the condition is called hypothyroidism. 

High T3-Uptake levels:

Abnormally high levels of T3-Uptake mean low TBG levels in the blood. Low TBG may be caused due to:

  • Calorie deficit diets.
  • Diabetes.
  • Alcohol intake.
  • Genetics.
  • Certain medications.

High T3-Uptake can also be a sign of hyperthyroidism, in which the thyroid gland produces abnormally high amounts of T3 and T4. 

Related Tests: Total T4 test, Total T3 test, TSH test.

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