TBG blood test
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).
How the Test is Performed
A blood sample is taken and then sent to a laboratory for testing.
Blood sample
Venipuncture is the collection of blood from a vein. It is most often done for laboratory testing.
Read Article Now Book Mark ArticleHow 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)
Acute intermittent porphyria
Porphyrias are a group of rare inherited disorders. An important part of hemoglobin, called heme, is not made properly. Hemoglobin is a protein in ...
Read Article Now Book Mark Article - Hypothyroidism (underactive thyroid)
Hypothyroidism
Hypothyroidism is a condition in which the thyroid gland does not make enough thyroid hormone. This condition is often called underactive thyroid....
Read Article Now Book Mark Article - Liver disease, such as hepatitis
Liver disease
The term "liver disease" applies to many conditions that stop the liver from working or prevent it from functioning well. Abdominal pain or swelling...
Read Article Now Book Mark Article - Pregnancy (TBG levels normally increased during pregnancy)
Note: TBG levels are normally high in newborns.
Decreased TBG levels may be due to:
- Acute illness
Acute
Acute means sudden. Acute symptoms appear, change, or worsen rapidly. It is the opposite of chronic.
Read Article Now Book Mark Article - Acromegaly (disorder caused by too much growth hormone)
Acromegaly
Acromegaly is a condition in which there is too much growth hormone (GH) in your body.
Read Article Now Book Mark Article - Hyperthyroidism (overactive thyroid)
Hyperthyroidism
Hyperthyroidism is a condition in which the thyroid gland makes too much thyroid hormone. The condition is often called overactive thyroid.
Read Article Now Book Mark Article - Malnutrition
Malnutrition
Malnutrition is the condition that occurs when your body does not get enough nutrients.
Read Article Now Book Mark Article - Nephrotic syndrome (symptoms that show kidney damage is present)
Nephrotic syndrome
Nephrotic syndrome is a group of symptoms and abnormal test results that include protein in the urine, low blood protein levels in the blood, high ch...
Read Article Now Book Mark Article - 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.
Free T4
T4 (thyroxine) is the main hormone produced by the thyroid gland. A laboratory test can be done to measure the amount of free T4 in your blood. Fre...
Read Article Now Book Mark ArticleRisks
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)
Hematoma
A bruise is an area of skin discoloration. A bruise occurs when small blood vessels break and leak their contents into the soft tissue beneath the s...
Read Article Now Book Mark Article - Infection (a slight risk any time the skin is broken)
Reviewed By
Sandeep K. Dhaliwal, MD, board-certified in Diabetes, Endocrinology, and Metabolism, Springfield, VA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
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.