TBG blood test

Definition

The TBG blood test measures the level of a protein that moves thyroid hormone throughout your body. This protein is called thyroxine-binding globulin (TBG).

Alternative Names

Serum thyroxine binding globulin; TBG level; Serum TBG level; Hypothyroidism - TBG; Hyperthyroidism - TBG; Underactive thyroid - TBG; Overactive thyroid - TBG thyroid hormone binding globulin

How the Test is Performed

A blood sample is taken and then sent to a laboratory for testing.

How to Prepare for the Test

Certain drugs and medicines can affect test results. Your health care provider may tell you to stop taking a certain medicine for a short time before the test. Never stop taking any medicine without first talking to your provider.

These medicines and drugs can increase TBG level:

• Estrogens, found in birth control pills and estrogen replacement therapy
• Heroin
• Methadone and opioids
• Phenothiazines (certain antipsychotic medicines)

The following medicines can decrease TBG levels:

• Depakote or depakene (also called valproic acid)
• Dilantin (also called phenytoin) and carbamazepine
• High doses of salicylates, including aspirin
• Male hormones, including androgens and testosterone
• Prednisone or other glucocorticoids (steroids)

How the Test will Feel

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or slight bruising. This soon goes away.

Why the Test is Performed

This test may be done to diagnose problems with your thyroid.

Normal Results

Normal range is 13 to 39 micrograms per deciliter (mcg/dL), or 0.13 to 0.39 milligrams per liter (mg/L).

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or may test different samples. Talk to your provider about the meaning of your specific test results.

What Abnormal Results Mean

An increased TBG level may be due to:

• Acute intermittent porphyria (a rare metabolic disorder)
• Hypothyroidism (underactive thyroid)
• Liver disease, such as hepatitis
• Pregnancy (TBG levels normally increased during pregnancy)

Note: TBG levels are normally high in newborns.

Decreased TBG levels may be due to:

• Acute illness
• Acromegaly (disorder caused by too much growth hormone)
• Hyperthyroidism (overactive thyroid)
• Malnutrition
• Nephrotic syndrome (symptoms that show kidney damage is present)
• Stress from surgery

High or low TBG levels affect the relationship between total T4 and free T4 blood tests. A change in TBG blood levels can alter the appropriate dose of levothyroxine replacement for persons with hypothyroidism.

Risks

There is little risk involved with having your blood taken. Veins and arteries vary in size from one person to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks of having blood drawn are slight, but may include:

• Excessive bleeding
• Fainting or feeling lightheaded
• Multiple punctures to locate veins
• Hematoma (blood buildup under the skin)
• Infection (a slight risk any time the skin is broken)

References

Faix JD. Thyroid function testing (thyrotropin, triiodothyronine, and thyroxine). In: Robertson RP, ed. DeGroot's Endocrinology. 8th ed. Philadelphia, PA: Elsevier; 2023:chap 68.

Guber HA, Oprea M, Russell YX. Evaluation of endocrine function. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 25.

Pearce EN, Hollenberg AN. Thyroid. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 207.

Salvatore D, Cohen R, Kopp PA, Larsen PR. Thyroid pathophysiology and diagnostic evaluation. In: Melmed S, Auchus RJ, Goldfine AB, Koenig RJ, Rosen CJ, eds. Williams Textbook of Endocrinology. 14th ed. Philadelphia, PA: Elsevier; 2020:chap 11.

Wassner AJ, Smith JR. Disorders of thyroxine-binding globulin. In: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds. Nelson Textbook of Pediatrics. 21st ed. Philadelphia, PA: Elsevier; 2020:chap 580.