Posted October 1, 2022 by Anusha ‐ 4 min read
A T4 test is used to check how well your thyroid is working. It's also used to help diagnose and monitor thyroid disease and conditions that may affect your thyroid
Other names of T4 test
Total T4 concentration
Free T4 concentration
Thyroxine test by equilibrium dialysis
What is a Thyroxine (T4) Test?
A thyroxine test is a blood test that helps diagnose thyroid conditions.
The thyroid is a small, butterfly-shaped gland at the base of your throat.
Your thyroid makes hormones that control the way your body uses energy.
These hormones affect your weight, heart, body temperature, muscle strength, and even your mood.
In children, thyroid hormones affect growth, too.
Thyroxine, also known as T4, is a type of thyroid hormone.
A T4 test measures the level of T4 in your blood.
Too much or too little T4 can be a sign of thyroid disease.
There are two forms of T4 in your blood:
Free T4 is the active form of thyroxine hormone that enters your tissues where it’s needed.
Bound T4 is thyroxine that attaches or binds to certain proteins which prevent it from entering your tissues.
T4 levels can be measured with either a free T4 test or a total T4 test:
A free T4 test directly measures the amount of free T4 in your blood. Medical experts believe this test provides more accurate information than a total T4 test, so it’s used more often.
A total T4 test measures free and bound T4 together. A calculation may be done to figure out how much T4 is free. This is a less accurate way to check how well your thyroid is working.
Procedure of T4 test
- Talk to your doctor about any medication, vitamins, or supplements you’re taking.
- Medications for seizures, heart conditions, and birth control may affect the results of the test.
- If you take a biotin supplement, it may cause a false-positive test result, showing that you don’t have a thyroid problem.
- Some infections may also affect your test results.
- This includes chronic diseases like kidney failure or cirrhosis.
- Cirrhosis is scarring on your liver which can be caused by different diseases and conditions, including hepatitis and alcoholism.
- If you have a short-term infection, your doctor may want to wait to do the test until you feel better.
What is it used for?
A T4 test is used to check how well your thyroid is working.
It’s also used to help diagnose and monitor thyroid disease and conditions that may affect your thyroid, including:
Hypothyroidism, or underactive thyroid, which is when your thyroid gland doesn’t make enough thyroid hormone to meet your body’s needs
Hyperthyroidism, or overactive thyroid, which is when your thyroid gland makes more thyroid hormone than your body needs
Disorders of the pituitary gland, which is a gland at the base of your brain that makes TSH (thyroid stimulating hormone). TSH controls how much T4 your thyroid gland makes. If the results from a TSH test aren’t normal, a T4 test can help diagnose whether the problem is with your thyroid or your pituitary gland.
What do the results mean?
If you had a free T4 test, your test results may be reported as “free T4.” If you had a total T4 test, your results may be reported as
free T4 index (FTI). To fully understand your results, your provider will often need to compare them with the results of other thyroid tests.
In general, T4 results that are lower than normal may be a sign of:
- Hypothyroidism, which may be caused by Hashimoto disease or another medical condition that causes your thyroid to make too little T4
- Thyroiditis (thyroid inflammation)
- Side effects from certain drugs
- Congenital hypothyroidism (a condition that affects babies from birth)
In general, T4 results that are higher than normal may be a sign of:
Hyperthyroidism, which may be caused by Graves disease or another medical condition that causes your thyroid to make too much T4
Thyroiditis (thyroid inflammation)
Toxic goiter (an enlarged thyroid with areas that make extra thyroid hormone)
Toxic thyroid nodule (a single lump on the thyroid that makes extra thyroid hormone)
Too much iodine in your diet
A noncancerous (not cancer) tumor in your pituitary gland
Risks of T4 test
Drawing a sample of blood for the T4 blood test is similar to other blood labs you may have done. You get a small needle prick to extract blood, and then the technician covers the area with gauze and a bandage.
Side effects of the test may include:
Pain following the needle stick
Infection if the area isn’t kept clean
Bruising if your blood vessel bursts
Dizziness or fainting
Feeling a lump under your skin