What is a thyroxine (T4) test?
A thyroxine test helps diagnose disorders of the thyroid. The thyroid is a small, butterfly-shaped gland located near the throat. Your thyroid makes hormones that regulate the way your body uses energy. It also plays an important role in regulating your weight, body temperature, muscle strength, and even your mood. Thyroxine, also known as T4, is a type of thyroid hormone. This test measures the level of T4 in your blood. Too much or too little T4 can indicate thyroid disease.
The T4 hormone comes in two forms:
- Free T4, which enters the body tissues where it's needed
- Bound T4, which attaches to proteins, preventing it from entering body tissues
A test that measures both free and bound T4 is called a total T4 test. Other tests measure just free T4. A free T4 test is considered more accurate than a total T4 test for checking thyroid function.
Other names: free thyroxine, free T4, total T4 concentration, thyroxine screen, free T4 concentration
What is it used for?
A T4 test is used to evaluate thyroid function and diagnose thyroid disease.
Why do I need a thyroxine test?
Thyroid disease is much more common in women and most often occurs under the age of 40. It also tend to run in families. You may need a thyroxine test if a family member has ever had thyroid disease or if you have symptoms of having too much thyroid hormone in your blood, a condition called hyperthyroidism, or symptoms of having too little thyroid hormone, a condition called hypothyroidism.
Symptoms of hyperthyroidism, also known as overactive thyroid, include:
- Anxiety
- Weight loss
- Tremors in the hands
- Increased heart rate
- Puffiness
- Bulging of the eyes
- Trouble sleeping
Symptoms of hypothyroidism, also known as underactive thyroid, include:
- Weight gain
- Fatigue
- Hair loss
- Low tolerance for cold temperatures
- Irregular menstrual periods
- Constipation
What happens during a thyroxine test?
A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.
Will I need to do anything to prepare for the test?
You don't need any special preparations for a thyroxine blood test. If your health care provider has ordered more tests on your blood sample, you may need to fast (not eat or drink) for several hours before the test. Your health care provider will let you know if there are any special instructions to follow.
Are there any risks to the test?
There is very little risk to having a blood test. You may have slight pain or bruise at the spot where the needle was put in, but most symptoms go away quickly.
