What are the other names for the test?
Free thyroxine, Free T4, T4 index, Free T4 concentration, Thyroxine test by equilibrium dialysis, Total T4 concentration, Thyroxine screen
What is a T4 (Thyroxine) test?
A T4 test is a blood test that measures the thyroxine hormone in your blood. A thyroxine test diagnoses how well your thyroid is working. T4 is commonly known as thyroxine, a thyroid hormone. A low or high level of T in your blood may indicate thyroid disease. A thyroid is a small shaped-gland-like butterfly located in front of your throat. Your thyroid develops hormones, which provides a signal to your body when to utilize the energy. These hormones help to control your
- Body temperature
- Strength of the muscle
- Brain development
- Nervous system
There are two types of T4 hormone
Free T4 - It is the most active type of thyroxine hormone that comes into your body’s tissue when there is a requirement for it.
Bound T4 - This type of thyroxine hormone combines with specific proteins that protect it to enter into your tissues.
Your provider can measure T4 hormone by free T4 test and a total T4 test.
A free T4 test is a direct method to determine the free T4 levels in your blood. This test gives more accurate results compared to a total T4 test.
A total T4 test determines free and bound T4 simultaneously. Your provider will do measurement to find the availability of free T4. It is a less accurate method to diagnose your thyroid condition.
What is the purpose of T4 test?
A thyroxine test indicates to your provider about the conditions of your thyroid. A T4 test in specific cases, often part of a combination of tests known as thyroid panel. It contains a TSH test besides other thyroid tests. This test monitors and diagnoses the thyroid disease, such as
- Hypothyroidism - It is a condition of underactive thyroid in which your thyroid gland does not produce enough thyroid hormone.
- Hyperthyroidism - It is a condition of overactive thyroid in which your thyroid gland produces more thyroid hormone than the need.
- Problems in the pituitary gland - It is a gland located at the center of your brain. It develops TSH (thyroid-stimulating hormone) that regulates the amount of T4 developed by your thyroid gland. If your TSH test results are not in the ideal range, T4 test helps to diagnose and detect the disorder of your pituitary gland or your thyroid.
In general, a TSH test performs first before this test. If the results of the TSH test are not in the recommended range, a T4 test may be conducted. If you have a medical or a family history of thyroid disease, you may need to order a thyroxine test. Your healthcare provider recommends this test if you experience symptoms of hypothyroidism or hyperthyroidism. Symptoms of hypothyroidism include
- Weight gain
- Pain in the joints and muscles
- Lack of tolerance to cold temperature
- Decreased heart rate
- Irregular menstrual period
- Fertility problems
- Thin and/or dry hair
- Dry skin
- Anxiety and nervousness
- An enlarged thyroid gland known as Goiter
- Unexpected weight loss when you have an increase in your appetite
- Muscle weakness
- Shivering in the hands
- Lack of tolerance to heat or sweating
- Increase bowel movements
- Disturbance in the rate of heartbeat also called Arrhythmia
- Difficulty sleeping
Your healthcare provider asks for a blood sample for a thyroxine test. Small needle will insert in a vein in your arm. Small quantity of blood will be collected in a test tube. A bandage applies to your vein. The test does not take time greater than 5 minutes.
How do I need to prepare for the test?
You may need to stop taking certain medicines that can affect your test results. Your provider will inform you about any specific instructions for the test until you do not need to prepare for the test. If you may need to order additional blood tests, you have to keep fast for at least 7 to 8 hours before the test. Your provider may give special instruction if needed.
Is there any risk in the test?
A T4 test does not have a risk. You may feel a little pain or bruising at the spot of your vein when your provider injects a needle. But these symptoms do not last for long.
What is the recommended range for the test?
The test results depend on the person’s age, gender, medical history, and other factors. Your test results may slightly differ among laboratories. Each lab may use different measurement methods. You can schedule an online appointment with your doctor about the understanding of your test results.
The ideal range for this test is between 0.8 to 1.8 nanograms per deciliter (ng/dL). A higher level of T4 than recommended indicates that your thyroid produces a large amount of T4 hormones in your body. The other signs and symptoms for higher T4 level include
- Hyperthyroidism that may occur due to Graves’ disease or a medical condition that creates a problem for your thyroid to develop large amount of T4
- Thyroiditis, a condition in which Inflammation (burning) of thyroid take place
- Toxic thyroid nodule, an individual bump on your thyroid that responsible to produce more thyroid hormone
- Toxic goiter (an enlargement of thyroid that creates large quantity of thyroid hormone)
- Intake of large amount of iodine in your diet
- Occurrence of large amount of noncancerous (non-cancer) tumour in your pituitary gland
- Hypothyroidism that may occur due to Hashimoto disease or a medical condition that creates a problem for your thyroid to develop less amount of T4
- Use of specific medicines that have side effects
- Congenital hypothyroidism, a condition that creates problems for babies during their birth)
What additional tests may I need to conduct along with this test?
Your provider may ask for a T3 and TSH hormone test. You may need to take additional blood tests to observe the specific antithyroid antibodies in your blood.
What factors may affect the test results?
Factors that may change your test results include the use of specific medicine like phenobarbital. Some chronic illnesses like renal failure and cirrhosis of the liver may also affect the test.
What else I need to know about a thyroxine test?
Hypothyroidism has more chances to develop during or after pregnancy. If you have been diagnosed with thyroid conditions during pregnancy, your healthcare provider will observe your condition as your baby is born. T4 test usually helps to screen for congenital hypothyroidism in newborn babies.