1
2
5
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Blood test
Blood test

Serum TBG level
Definition:
Serum TBG level is a test to evaluate thyroid function by measuring the level of thyroxine binding globulin (TBG) in the blood. Thyroxine binding globulin is a protein that transports thyroid hormone around your body.

Alternative Names:
Serum thyroxine binding globulin; TBG level

How the test is performed:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The area where blood is to be drawn is cleaned with antiseptic, and a tourniquet (a tight elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend, or fill with blood. A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. Once the blood has been collected, the tourniquet is removed to restore circulation, the needle is removed, and the puncture site is covered to stop any bleeding.

The sample is then taken to the laboratory where it is evaluated by special laboratory tests, such as electrophoresis or radioimmunoassay.
How to prepare for the test:

Several medications and conditions can interfere with TBG test results.

If you take any of the following, your TBG results may be elevated:

  • Estrogens, found in oral contraceptives or estrogen replacement therapy
  • Phenothiazines
  • Heroin or methadone

If you have one of the following conditions, your TBG results may also be elevated:

  • Severe liver disease
  • Acute intermittent porphyria
  • HIV

If you take any of the following, your TBG levels may be reduced:

  • High doses of salicylates, such as aspirin
  • Male hormones, such as androgens and testosterone
  • Depakote or Depakene (also called valproic acid)
  • Dilantin (also called phenytoin)
  • Prednisone

If you have one of the following conditions, your TBG results may also be reduced:

  • Nephrotic syndrome, or kidney failure
  • Other conditions that cause low albumin levels, such as liver disease

If you take any of these medications, or have any of these conditions, inform your doctor before the test, since you may need to withhold your medication, or have another test.

If your child is to have this test performed it may be helpful to explain how the test will feel, and even practice or demonstrate on a doll. The more familiar your child is with what will happen to him/her, and the purpose for the procedure, the less anxiety he/she will feel.
How the test will feel:
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed:
Your doctor may use this test to identify abnormalities with your thyroid binding globulin, or to assist in diagnosing other thyroid disorders such as hypothyroidism.
Normal Values:
Normal values vary somewhat from laboratory to laboratory, and depend on which method is used to evaluate the sample. If electrophoresis is used, results are reported in terms of T4 binding capacity (the binding capacity of the thyroid hormone T4), and normal values may range from 10 mcg/100 ml to 24 mcg/100 ml.

If radioimmunoassay is used to evaluate the sample, then a normal range of 1.3 to 2.0 mcg/100 ml can be expected.

Note: mcg/ml = microgram per milliliter
What abnormal results mean:
Other than problems with your thyroid gland, elevated TBG levels may indicate: Decreased TBG levels may indicate:
What the risks are:
Risks associated with venipuncture are slight:
  • Excessive bleeding.
  • Fainting or feeling lightheaded.
  • Hematoma, or blood accumulating under the skin.
  • Infection (a slight risk any time the skin is broken).
  • The person taking the blood may have to make multiple punctures to locate veins.
Special considerations:
Veins and arteries vary in size from one patient 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.
Review Date: 10/22/2001
Reviewed By: Dr. Jacqueline Brooks, MBBCh, freelance medical writer for A.D.A.M., Inc.
A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org). URAC's accreditation program is the first of its kind, requiring compliance with 53 standards of quality and accountability, verified by independent audit. A.D.A.M. is among the first to achieve this important distinction for online health information and services. A.D.A.M. is also a founding member of Hi-Ethics (www.hiethics.com) and subscribes to the principles of the Health on the Net Foundation (www.hon.ch).

The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed physician should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. Copyright 2003 A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.
adam.com