What is the T3 Resin Uptake Test?

What to expect when undergoing this test

The T3 resin uptake (T3RU) test is a test of thyroid-related proteins in the blood. It's used to help doctors interpret the results of other thyroid tests, namely the T3 and T4 tests, which show how much thyroid hormone is available to the body in free form. However, it's used less than it used to be due to the development of the free T4 and serum TBG (thyroxine-binding globulin) tests.

Alternate names for this test include resin t3 update, T3 uptake, and thyroid hormone-binding ratio.

Purpose of Test

Your thyroid is a gland that sits in the front of your throat and is shaped like a butterfly. It produces hormones and regulates a lot of important functions in your body, including energy use, weight, mood, and body temperature.

Two key thyroid hormones are:

  • T3 (triiodothyronine)
  • T4 (thyroxine)

Both exist in two forms. A protein in your blood called thyroxine-binding globulin (TBG) binds to the more abundant forms of both T3 and T4 and helps it move through your bloodstream. The other types are left circulating without this protein and are called "free" T3 and T4. The free forms are available to help regulate your bodily functions.

The T3RU test is an indirect measurement of how much TBG binding is going on. It's helpful for your doctor to know this information because very often, abnormal T4 test results can be an indication of TBG imbalance rather than a problem with the production of T4 itself. That can lead to misdiagnosis of a thyroid problem.

Your doctor may order a T3RU test if you have symptoms that may be caused by hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid), especially if you have other risk factors, which include:

  • Being female
  • Being under age 40
  • Family members with thyroid disorders

This test is intended to be done alongside T3 and T4 tests to help your doctor get a complete picture of your thyroid function.

It may also be done to check for:

The T3RU and other thyroid tests are done on blood samples. Taking blood samples generally is a quick, simple, and low-risk procedure.

Risks

For most people, the only risk from a blood test is mild pain or light bruising that goes away quickly.

Some people, especially those with a fear of needles, may experience side effects such as dizziness, nausea, or ringing in the ears during or right after having their blood drawn. If you experience this, let someone know. Most places where blood is drawn are equipped to deal with this kind of reaction. You'll usually be asked to drink some water and lie down until you feel better, which normally takes just a few minutes.

If you know you're likely to have a negative reaction, it's a good idea to arrange for someone to give you a ride.

To Minimize Risks

Let the nurse or phlebotomist drawing your blood know if:

  • You've had bad reactions to blood draws in the past
  • You have a bleeding disorder or are taking blood-thinning medications
  • Your skin tears or bruises easily

They can take steps to lessen any risk these factors create.

Before the Test

The T3RU blood test requires fasting beforehand, meaning you can't eat for 10 to 12 hours before the blood draw. Your doctor or the testing facility should give you instructions. Make sure you follow them as closely as possible. Because of the fasting requirement, this test is most often performed in the morning.

Be sure to take your insurance card with you as well as any written orders your doctor may have provided.

Talk to your doctor about any medications you're taking, as some of them may alter your test results. Don't stop taking anything unless your doctor tells you to.

Medications that may raise T3RU levels include:

  • Anabolic steroids
  • Heparin
  • Phenytoin
  • High doses of salicylates (such as aspirin)
  • Warfarin

Drugs that can lower T3RU levels include:

  • Antithyroid medicines
  • Birth control pills
  • Clofibrate
  • Estrogen
  • Thiazides

Over-the-counter medicines and supplements may also interfere with your results, so make sure you tell your doctor about everything you're taking.

Pregnancy can throw off results as well, so if you're pregnant, make sure you're doctor knows.

Timing and Location

A blood draw should only take a few minutes. Some doctor's offices may draw the blood while you're there for an appointment. In that case, your appointment might be slightly longer than is typical.

Going to a lab for the test involves more time. If you have a scheduled appointment for your test, arrive early enough to check in and take care of co-pays. If you're nervous about the blood draw, you may also want to give yourself a few minutes to sit and relax after you get there.

Labs can be busy and wait times may be hard to predict. If you're dropping in, it's a good idea to call ahead to see how long the wait is. Certain times of day, especially early mornings, may be better than others.

What to Wear

You shouldn't need to change out of your regular clothes for this test. However, make sure you can expose most of your arm without too much trouble.

In case you get dizzy afterward, flat shoes and non-restrictive clothing may be good choices.

Food and Drink

Remember to follow any fasting instructions you're given.

Any time you're having blood drawn, you want to be well hydrated. It makes your veins easier to find, which helps avoid problems with inserting the needle.

It's a good idea to take a snack with you so you can eat right after the test.

Cost and Health Insurance

Most of the time, insurance does cover thyroid-function tests that are deemed medically necessary. You should check with your insurance company ahead of time, though, so you're aware of any expenses you could face.

Without insurance, laboratory costs for a package of thyroid tests, which may or may not include a T3RU, typically costs between $150 and $180 dollars. If a radioactive iodine uptake test is also ordered, it could push the total cost up over $500.

Your insurance company, doctor's office, and lab should be able to help you figure out what, if any, costs you'll need to cover.

During the Test

Your blood will probably be drawn by a nurse if it's done in a doctor's office, or by a phlebotomist if it's done at a lab. Before the test, they may ask you to confirm certain information, such as your name, birth date, the doctor who ordered the test, and what you're being tested for. This is to ensure they're doing the correct test(s) on the correct person.

Pre-Test

You'll be asked to expose your arm, and then the nurse or phlebotomist will clean the insertion site with alcohol. Next, they'll tie a band around your upper arm to trap the blood and get your veins to stand out. If this isn't working well, they may ask you to pump your fist.

Throughout the Test

Once they find a good vein, they'll insert the needle. It'll probably sting just a little, but this usually hurts less than a shot, since nothing is being injected.

They'll release the band to get the blood flowing, and blood will start flowing through the needle to an attached vial. Depending on how many tests are ordered, you may need to fill up more than one vial.

Once enough blood is drawn, they'll withdraw the needle and bandage the insertion site.

Post-Test

You should be able to leave right away after your blood is drawn unless you have a negative reaction. Even then, it usually only takes a few minutes of rest before it's okay for you to leave.

After the Test

You may notice a little tenderness and possibly bruising around the insertion site. This should go away quickly.

If you have any problems or questions after a blood test, contact your doctor.

Managing Side Effects

If pain at the site bothers you, ice or an over-the-counter pain medication should help. You shouldn't have any other lingering effects.

Interpreting Results

The normal ranges for adults are generally considered to be 24 percent to 37 percent.

Not every lab uses the same ranges or measurements, so ask your doctor if you're not sure what your results mean.

If your results are higher than normal, it could indicate:

  • Hyperthyroidism
  • Protein malnutrition
  • Kidney problems

Results that are lower than normal may indicate:

Some people have an inherited condition that involves high TBG levels despite normal thyroid function.

Follow-Up

After your results are in, your doctor may want you to come in to discuss treatment options or further testing, depending on whether a diagnosis can be made.

If you don't hear back about your results for a long time, or if you don't understand the results, contact your doctor's office.

If you do become diagnosed with a thyroid disorder, then it's time to start considering your treatment options.

A Word From Verywell

No one wants to have a chronic condition like a thyroid disorder, but sometimes a diagnosis can actually be a relief because you finally know what's going on. Keep in mind that thyroid disorders are common and highly treatable. Many people manage them by taking medication and maybe making some dietary changes.

Once you start treatment, it shouldn't take long for you to start feeling better.

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Article Sources

  • Dunlap DB. Thyroid Function Tests. In: Walker HK, Hall WD, Hurst JW, editors. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. Boston: Butterworths; 1990. Chapter 142.Available from: https://www.ncbi.nlm.nih.gov/books/NBK249/