What Is the Thyroid-Binding Globulin Test?

Thyroid-binding globulin (TBG) is a protein that's produced by the liver. Its purpose is to bind to the thyroid hormones, thyroxine (T4) and triiodothyronine (T3), produced by the thyroid gland and carry them through your bloodstream so they can regulate your metabolism and perform other important functions. The serum TBG level test is a blood test—sometimes referred to as the thyroxine-binding globulin test—that measures the amount of the TBG protein in your blood. Thyroid hormone that isn't bound to TBG is called "free" T3 or T4.

Purpose of Test

Healthcare providers use the results of a TBG test to help them assess thyroid problems, including hypothyroidism (low thyroid hormone levels) and hyperthyroidism (high thyroid hormone levels), as well as to identify other conditions that may be altering the levels or activity of the thyroid hormones in your blood.

When your thyroid hormone levels are high or low, it can be due to several different conditions—some that involve the thyroid gland's function and others that don't—and the TBG level can help point toward which one may be responsible.

This test doesn't paint a complete picture of thyroid function (or any other disease or condition). It must be considered along with other tests of thyroid function, which may include:

• Thyroid-stimulating hormone (TSH) test
• Free T4 test (also called free thyroxine)
• Total T4 test (total thyroxine)
• Free T3 test (free triiodothyronine)
• Total T3 test (total triiodothyronine)

TBG levels don't mean much in cases of hypothyroidism or hyperthyroidism due to actual dysfunction of the thyroid gland. They do, however, become especially important if you have abnormal levels of T3 or T4 but no symptoms of thyroid dysregulation.

If the TBG level is high, for example, the TBG binds to more thyroid hormone, and that leaves less of the free hormone circulating in the blood. That leads the body to put out more thyroid-stimulating hormone, which leads to the production of more thyroid hormone. Thus, the total thyroid hormone level will be elevated even though the person does not have hyperthyroidism.

Elevated TBG levels may be caused by:

• Hypothyroidism
• Liver disease
• Pregnancy
• Acute intermittent porphyria
• Genetics

Low TBG levels may be caused by:

• Hyperthyroidism
• Kidney disease
• Liver disease
• Severe systemic illness
• Acromegaly
• Malnutrition
• Certain medications
• Cushing's syndrome

Symptoms that may indicate a thyroid problem and prompt your healthcare provider to order this test include:

• Constipation or diarrhea
• Insomnia or other changes in your sleep pattern or quality
• Dry or puffy skin
• Dry, irritated, puffy, or bulging eyes
• Fatigue
• Weakness
• Hair loss
• Tremors in your hands
• Increased heart rate
• Sensitivity to cold or light
• Menstrual cycle irregularities
• Unexplained weight gain or loss

Risks

The TBG test is done by drawing blood to be tested in the lab. A blood draw is a simple, routine procedure that carries very little risk. The minor risks include:

• A small bruise at the needle insertion site, which should go away quickly
• Minor bleeding
• Inflammation of the vein (rare)
• Infection (rare)

If you have a fear of needles, you may experience dizziness, nausea, or ringing in the ears during or immediately following a blood draw. If this happens to you, tell the person drawing your blood. They'll likely have you drink some water and lie down for a few minutes.

If you have a history of negative reactions, you may want to arrange for someone to drive you to and from the test facility.

Let the person drawing your blood know beforehand 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 mitigate any risk these factors may create for you.

Before the Test

Before your TBG test, you may need to take a break from certain medications that can alter your levels, including:

• Aspirin
• Birth control pills
• Hormones
• Steroids
• Opioid pain killers
• Depakote
• Dilantin
• Phenothiazines
• Valproic acid

Timing

If you're fasting, you'll probably be told to go in for your blood draw first thing in the morning. If not, any time of the day should be fine.

The blood draw itself should only take a few minutes. If it's done outside of a healthcare provider's appointment, you may want to call the facility to see how long the wait is likely to be and if certain times are better than others.

Location

You may have your blood drawn in your healthcare provider's office or clinic, a lab, or a hospital.

What to Wear

Short sleeves or sleeves that are easy to push up above your elbow are preferable when you're getting a blood test.

Food and Drink

Fasting isn't typically required for a TBG test. However, you may need to avoid food and drink before other tests that may be ordered at the same time; be sure to follow the instructions you're given so you don't risk throwing off the results of your tests.

If you haven't been told to limit your water intake for another test, make sure to arrive for your blood test well hydrated. Dehydration can make needle insertion harder.

Cost and Health Insurance

The cost of a TBG test can vary, depending on where you have it done. Available pricing information ranges from $80 to about $150.

If you have insurance, check to see whether this test is covered and what, if any, out-of-pocket costs you could face.

What to Bring

If your healthcare provider gave you written orders, take them with you. Also, be sure you have a current insurance card with you. Some facilities may ask for valid identification, such as a driver's license, as well.

You shouldn't need to bring anything else with you to the blood draw, other than possibly something to entertain you while you wait.

During the Test

Typically, your blood will be drawn by a nurse or phlebotomist. They may ask you to confirm information such as your name, birth date, the healthcare provider who's ordering the test, and what test(s) you've been sent there for. This is to help avoid mistakes.

Pre-Test

You'll be asked to expose your arm, the insertion spot will be cleaned with alcohol, and the nurse or phlebotomist will tie a band around your upper arm to get the veins to stand out. If your veins don't stand out well, you may be asked to pump your fist.

Throughout the Test

Once a good vein is found, the needle will be inserted. It might hurt for a couple of seconds. The band will be released to get the blood flowing into the vial attached to the needle. Depending on how many tests are ordered, you may need to fill up two or more vials.

After enough blood has been drawn, the needle will be withdrawn and the insertion site will be bandaged.

Post-Test

Most of the time, you can leave right after the test is over. If you have any kind of negative reaction, speak up so you can get the proper care. Usually, even with a bad reaction, people are OK after a few minutes.

After the Test

The insertion site might be a little soreness, but that shouldn't last long. If you have any problems or questions, contact your healthcare provider's office.

Managing Side Effects

Ice or over-the-counter pain medications can help if you have any pain or inflammation. You shouldn't have any other lingering effects.

Interpreting Results

Results depend on which technique the laboratory uses to evaluate your blood sample.

Electrophoresis: In this method, an electrical current is run through your blood serum. Normal results from this method range from 10 to 24 milligrams (mg)/100 milliliters (mL).

Radioimmunoassay: This method involves a radioactive isotope-containing antibody that attaches to TBG, and then the radiation level is measured. The normal range from this method is from 1.3 to 2.0 mg/100mL.

If TBG results are normal but thyroid hormone levels are low, the diagnosis is typically hypothyroidism. If TBG is normal but thyroid hormone levels are high, the likely diagnosis is hyperthyroidism.

If TBG is abnormal, your healthcare provider will need to look at the results of all thyroid tests and may need additional tests, as well, in order to determine what's going on.

A Word From Verywell

Because so many things can cause abnormalities in your TBG level, your follow-up depends on the results of all the tests your healthcare provider has ordered and what, if any, diagnosis was made. Your practitioner should be able to tell you what the results mean in the context of your overall health as well as what treatment, if any, is indicated.