Thyroxine T4 Blood Test

What is Thyroxine

Thyroxine is a hormone made by the thyroid gland. This Thyroxine hormone is also known as T4.

T4 hormone is of two two types- bound T4 and free T4.

Bound T4 is the T4 hormone that are attached to the proteins. Free T4 are the unbound T4 that are not attached to proteins. 

Most of the T4 in our body remains as bound T4, only a small portion of T4 remains as free form. This free T4 is acts on our body cells and tissues.

T4 test is important to check the thyroid status. A low T4 usually means hypothyroidism or underactive thyroid. Similarly a high T4 means hyperthyroidism or overactive thyroid.

What is T4 blood Test

T4 Blood Test is the test that measures the level of thyroxine hormone, also called the T4 hormone in our blood. As mentioned earlier, as T4 available in two forms- free T4 and bound T4, so T4 test can also be of two types.

  • Free T4 Test – This test measures the level of free T4 that are circulating in our blood.
  • Total T4 Test – This test measures the level of protein bound T4 that are circulating in our blood through protein and the free T4 present in blood. So it is the total of free T4 and bound T4. 99% of T4 in our body are in this bound form.

T4 Blood Test Purpose

Purpose of the T4 test are the following 

  1. To diagnose thyroid disorder ( both hypothyroidism and hyperthyroidism)
  2. To follow up the thyroid if the TSH test result is abnormal
  3. To monitor T4 levels in body when the patient is under thyroid replacement therapy
  4. To screen for underactive thyroid in newborn.
  5. To evaluate other thyroid disorders such as thyroid nodules, goiters etc.
  6. To diagnose and evaluate pituitary gland disorder and hypothalamus disorder.

Why the T4 blood test is performed :

The test is needed to perform if there is

  1. Abnormal test results of other thyroid tests such as T3 and TSH
  2. Symptoms of underactive thyroid 
  3. Symptoms of overactive thyroid
  4. Underactive pituitary gland 
  5. Nodule or lump in the thyroid gland
  6. Enlarged thyroid gland
  7. Problems becoming pregnant 
  8. Monitoring people with already diagnosed thyroid disorder.

T4 Blood Test Importance

Importance of T4 test are the following-

  1. To determine the appropriate treatment of patient with a thyroid disorder
  2. To know the exact dose of thyroid medicine based on T4, T3 and TSH test result
  3. To check the response of the thyroid treatment 
  4. To monitor the treatment outcome
  5. To determine if any change of treatment is needed or not based on patient response to thyroid treatment
  6. To know if any change of thyroid medicine is needed or not. Also, it helps to determine if any change of dose of medicine is needed or not.

T4 Blood Test Procedure

T4 test is done just like other thyroid tests such as T3, TSH test. T4 test is also a blood test. For this test, blood is drawn from vein ( preferably vein from arm). T4 test result is prepared based on this blood sample. Usually no special precautions is needed for this test. 

If you are taking any supplements such as biotin, you may need discontinuation of the supplement for few days before the test. Do not discontinue any medicine or supplement without consulting with your doctor. Follow your doctors instructions.

T4 Blood Test Measures

T4, also known as thyroxine, is an important hormone of thyroid gland. T4 has four iodine atoms attached to it.

There are various functions of thyroxine hormone. It not only helps for a proper brain development, it is also involved for many vital body functions such as regulating weight, muscle strength, body temperature and many more functions. Without this hormone, a healthy body is not possible.

Body has large stores of T4, this T4 are circulating in our blood. But, this large store of T4 are the bound form of T4. So this bound T4 can not be used for immediate purpose as only free T4 is responsible for all body functioning.

Free T4 test measures the unbound T4 that are not attached to proteins. This is the active form of T4.

Total T4 test measures both the unbound and bound T4 circulating in the blood.

T4 Blood Test Levels

Normal range of  free T4 is- 0.8 to 1.8 ng/dL

Test results may also be described as free T4 or free thyroxine index (FTI). Free thyroxine index means the test result is calculated based on a formula that compares total T4 and estimation of level of thyroid hormone proteins.

High T4 test result simply means hyperthyroidism or overactive thyroid and low T4 test result means hypothyroidism or underactive thyroid.

Normal T4 result does not mean that the thyroid gland is normally functioning. A normal T3 and normal TSH result along with normal T4 test result indicates normal functioning thyroid. Abnormal test result of any of these indicates that thyroid gland is not functioning properly.

Interpretation of free T4 test result with TSH test result

TSH LevelFree T4 LevelCorresponding Thyroid Condition
NormalNormalNormal Thyroid Function
HighNormalSubclinical Hypothyroidism
HighHighHyperthyroidism Related To Pituitary Gland
HighLowHypothyroidism Related To Thyroid Gland
LowNormalSubclinical Hyperthyroidism
LowHighHyperthyroidism Related To Thyroid Gland
LowLowHypothyroidism Related To Pituitary Gland
Free T4 Blood Test Result Meaning

Subclinical thyroid conditions may not give any symptoms. But it is very important to diagnose subclinical thyroid disorder because with time they can progress to hypothyroidism or hyperthyroidism.

T4 Blood Test Result Accuracy and Variations

Free T4 test results are usually accurate. Some factors can interfere with the test results such as

  1. Supplements or drugs
  2. Pregnancy 
  3. Very ill patients 
  4. Malnourished patients because of low levels of proteins with which T4 binds

Drugs that affect T4 Blood Test Result

There are certain drugs that affect T4 blood test results. Some of the drugs are

  1. Steroids
  2. Birth control pills ( oral contraceptives)
  3. Heart medicines
  4. Seizure medicines
  5. Aspirin
  6. Drugs containing hormone such as androgen and estrogen
  7. Some drugs for cancer treatment 
  8. Drugs that are used to treat thyroid disorder

Free T4 Test Vs. Total T4 Test

Most of the T4 is in the bound form, less then 1 percent of T4 is in the free form.

Total T4 blood test results are usually less reliable than free T4 blood test because bound T4 is dependent upon the protein level of our body to which T4 binds. Many factors can change the protein levels such as pregnancy, severe illness, oral contraceptives,corticosteroids, etc. So, based upon the protein level bound T4 test result can vary. As total T4 test measures both bound T4 and free T4, so test results are not that much reliable. For this reason, free T4 test result is considered to be accurate because the test result is not affected by the level of proteins.

Do I need follow up T4 Blood Test(s)?

Additional thyroid function tests may be needed to have a proper diagnosis.

  1. People with untreated subclinical hypothyroidism needs follow up with TSH test and free T4 test six months after the diagnosis and then annually.
  2. People with untreated subclinical hyperthyroidism needs follow up with TSH test, free T4 test and T3 test every six months.
  3. Newborn with screened T4 and TSH test needs a re test if test results are abnormal and should be evaluated by the doctor.
  4. People with already diagnosed thyroid disorder needs follow up based on the clinical condition. Usually, periodic thyroid function tests are needed and should be evaluated by the doctor to determine how well the thyroid gland is working.

T4 Blood Test High

High T4 test result is seen in the following conditions

  1. Overactive thyroid or hyperthyroidism 
  2. Thyroiditis 
  3. Toxic thyroid nodule or toxic goiter
  4. High protein levels in the blood
  5. Excessive iodine in body
  6. High dose of thyroid replacement drug 
  7. Germ cell tumor
  8. X-ray dyes ( as X-ray dye contain iodine, so a recent X-ray with dye can affect test result)

T4 Blood Test Low

Low T4 test result is seen in the following conditions 

  1. Hypothyroidism or underactive thyroid
  2. Pituitary disorder
  3. Illnesses ( usually severe and acute illness)
  4. Prolonged fasting
  5. Malnutrition 
  6. Iodine deficiency 
  7. Drugs that affect protein levels 


  1. Williams Textbook of Endocrinology E-Book
  2. Guyton and Hall Textbook of Medical Physiology
  3. Goldman-Cecil Medicine
  4. Davidson’s Principles and Practice of Medicine 23rd edition
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