Thyroxine binding globulin (TBG) test

Your doctor may order this test if he/she suspects that you have problems related to thyroid gland functioning. This test is helpful for the diagnosis of both hypothyroidism (an underactive thyroid) and hyperthyroidism (an overactive thyroid).

Symptoms of hypothyroidism include:

• Sensitivity to cold
• Fatigue
• Muscle cramps
• Dry, flaky skin
• Brittle hair and nails 
• Tingling sensation in the hands and fingers
• Weight gain
• Loss of libido 
• Irregular menstruation

Symptoms of hyperthyroidism are:

• Frequent mood changes 
• Sleeping difficulties 
• Sensitivity to heat
• Hyperactivity
• Anxiety, nervousness and irritability
• Persistent thirst
• Loss of libido 
• Fatigue

It is also one of the tests in the newborn screening for congenital hypothyroidism - inherited thyroid-binding globulin deficiency is one of the reasons for this condition.

No special preparations are required before this test. 

However, inform your doctor if you are taking any medications or drugs. This is because certain medicines may affect the test results. 

The following medications can increase thyroxine-binding globulin levels:

• Birth control pills that contain oestrogens
• Some antipsychotic drugs
• Methadone 
• Drugs such as heroin

Taking the following medications can decrease thyroxine-binding globulin levels:

• High doses of salicylates that include aspirin 
• Male hormones such as testosterone and androgens 
• Prednisone 
• Phenytoin 
• Depakote/Depakene 

Your doctor may ask you to skip a dose or discontinue these medicines. Do not stop taking any medications on your own.

A doctor or lab technician will use a sterile needle to withdraw a small amount of blood from a vein in your arm. 

There are no major side effects associated with this test. However, some people experience dizziness after the test or notice a temporary bruise at the needle insertion site. 

Normal results:

The normal range of thyroxine-binding globulin in the blood is as follows:

• Males: 12-26 mcg/mL
• Females: 11-27 mcg/mL

Abnormal results:

Higher than normal level of thyroxine-binding globulin can be attributed to:

• Hypothyroidism 
• Liver disease 
• Acute intermittent porphyria (a metabolic disorder)
• Pregnancy 
• Using oestrogen-based birth control methods

Thyroxine-binding globulin levels may also be high in newborns.

Lower than normal levels of thyroxine-binding globulin could be due to:

• Hyperthyroidism 
• Malnutrition 
• Kidney conditions
• Acromegaly (a disorder caused by the excessive production of growth hormone)
• Inherited thyroxine-binding globulin deficiency

Thyroxine-binding globulin levels may also be lower than normal after surgery.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.