TSH is a hormone. Full form of TSH is thyroid stimulating hormone. This hormone gives signal to thyroid gland to produce more thyroid hormone (T3 and T4). So, when there is more thyroid hormone (T3 and T4) in blood, the level of TSH will be low and vice versa.

TSH Blood Test

TSH test is a blood test for measuring the levels of this hormone. TSH is made and secreted from pituitary gland of brain. When the thyroid levels in our blood is low, TSH is secreted more from pituitary. Similarly, when thyroid hormone levels are high in blood, pituitary secrets less TSH.

TSH blood test gives only information whether the blood level is high, low or normal. So, through this test one can know if the thyroid is underactive or hyperactive. TSH blood test does explain why the TSH level is high or low. So, to know the cause of thyroid disorder, additional tests are needed. Thus, TSH is a part of routine thyroid test.

The other thyroid tests that one may need additionally are

  1. T3 thyroid hormone test
  2. T4 thyroid hormone test
  3. Tests to diagnose autoimmune Graves’ disease that causes hyperthyroidism (thyroid antibody test)
  4. Tests to diagnose autoimmune Hashimoto’s thyroiditis that causes hypothyroidism ( thyroid antibody test)

TSH acts on the thyroid gland by binding the TSH receptor. But TSH is made by pituitary gland of brain. This pituitary gland controls many glands of our body through the production of many hormones. One of the gland that is controlled by pituitary gland is thyroid gland. Pituitary gland can sense when the T3 and T4 hormone levels are high or low. As a response, pituitary will produce less or more TSH for the stimulation of thyroid gland, so thyroid gland can produce the appropriate amount of hormone.

TSH Levels

High or low TSH occurs when thyroid gland is not functioning properly. High TSH level simply means that thyroid is underactive (hypothyroidism) and needs to be stimulated. Similarly, Low TSH level means thyroid is overactive, so no need to be stimulated or stimulated less as per hyperactivity of thyroid gland. This condition is known as hyperthyroidism.

Normal TSH level in children:

TSH level are highest at birth and as the age of child increases, the TSH level decrease gradually.

TSH Levels Chart

Thyroid ConditionsTSH Level ( mIU/L )
Normal 0.4-4
Hyperthyroidism 0 to 0.4
Mild hypothyroidism 4-10
Hypothyroidism Above 10
TSH Test Results

TSH Test Results

TSH test results can be normal, high or low. 

If your TSH blood test result is normal, it usually means your thyroid is functioning properly.

If your TSH blood test result is too high, it indicates that your thyroid is not secreting enough thyroid hormone. It occurs in the condition that is known as hypothyroidism.

If your TSH blood test result is too low, it indicates that your thyroid gland is producing too much thyroid hormone. This condition is known as hyperthyroidism.

TSH Test Results Variation Factors

The following factors can interfere with your TSH test result

  1. Age- TSH level tends to be higher in people who are above 80 years old. But, mostly this older people with slightly higher TSH level have no associated health problems.
  2. Severe illness- People with severe illness other than thyroid problems may temporarily have low TSH.
  3. Pregnancy- In the first trimester of pregnancy, TSH  commonly is slightly lower than normal, and slowly rise thereafter.
    • Pregnancy hormone increases the thyroid hormone in blood naturally because the thyroid hormones are vital for the proper development of fetal brain and nervous system. So, the levels of TSH in pregnancy decrease and thyroid hormones increase.
    • In pregnancy the lower range of TSH is decreased by approximately 0.1 to 0.2 mU/l and the upper range of TSH is decreased by approximately 0.5 to 1.0 mU/l
  4. Drugs – Some drugs can interfere with TSH blood test results, for example, dopamine, lithium, amiodarone, prednisone, potassium iodide, biotin.

TSH Test Reason

TSH blood test is done to check if thyroid gland is working properly or not and how well thyroid gland is working.

TSH blood test is needed when it is suspected that thyroid gland is not working properly that is either thyroid is producing and secreting too much thyroid hormone or thyroid is secreting too less thyroid hormone.  

When there is too much thyroid hormone in blood it is known as hyperthyroidism. Similarly, when there is too little thyroid hormone in blood it is known as hypothyroidism.

High TSH Levels Symptoms

High TSH level indicates that there is underactive thyroid, this condition is termed as Hypothyroidism. Hypothyroidism or underactive thyroid has the following symptoms 

  1. Cold temperature intolerance
  2. Irregular menstrual period
  3. Constipation 
  4. Hair loss ( thin, dry hair)
  5. Weight gain
  6. Fatigue or tiredness
  7. Depression
  8. Dry skin
  9. Slow heart rate
  10. Fertility problems 

Low TSH Levels Symptoms

Low TSH level indicates that there is overactive thyroid, this condition is commonly known as Hyperthyroidism. Hyperthyroidism or overactive thyroid has the following symptoms 

  1. Anxiety
  2. Increased heart rate
  3. Tremor in the hands
  4. Weight loss
  5. Irritability 
  6. Bulging eyes
  7. Puffiness
  8. Difficulty sleeping (insomnia)
  9. Loose, frequent bowel movement or diarrhoea 
  10. Intolerance to heat

TSH Test Purpose

Purpose of the TSH blood test are the following 

  1. To diagnose a thyroid disorder ( such as underactive or overactive thyroid)
  2. To  know the overall health condition
  3. To determine the appropriate treatment 
  4. To differentiate various medical conditions that has common symptoms
  5. To predict the treatment outcome
  6. To determine the áppropriate dose of medicine
  7. To know the response of treatment
  8. To determine if any change of dose or any change of treatment is needed or not
  9. To screen for thyroid disorder before the symptoms appear, especially in newborn.
  10. To evaluate the thyroid disorder such as goiter
  11. To rule out thyroid diseases such as Hashimoto’s thyroiditis, Graves’ disease,thyroid cancer.

TSH Test Importance

Importance of thyroid test are the following 

  1. To monitor the patient who has known thyroid disorder
  2. To know how well the patient is doing
  3. To prevent thyroid disorder related various health issues
  4. To control thyroid disorder related symptoms
  5. To prevent further damage due to thyroid disorder
  6. For early detection and treatment of thyroid disease before the symptoms appear.
  7. Some drugs can affect the thyroid gland functioning such as lithium. When one is taking lithium, thyroid may not function properly. So, it is very important to check the level of TSH before starting to take the medicine.

TSH Blood Test Procedure

Blood is drawn from vein (preferably vein from arm) and based on this blood sample, the test report will come. No special precautions is needed for this test. No dietary restrictions or fluid restrictions is needed. You can do the test any time in a day. Your doctor will let you know if any special instructions you need to follow.

For example, if you are taking vitamin B7 or biotin supplements, it can interfere with the test result. So for the accuracy of TSH test report, the supplements needs to be discontinued at least two days before the TSH test.

Normal TSH Levels

TSH test results are reported in milli international units per liter (mIU/L). Normal range of TSH is controversial. But mostly the range of TSH that is considered is 0.4 to 4 mIU/L for an adult.

So, Reference range of TSH for an adult is 0.4 to 4 mIU/L. But ranges can vary from laboratory to laboratory.

High TSH Levels

High levels of TSH means that one has underactive thyroid. It can be due to

  1. Hashimoto’s thyroiditis 
  2. Thyroiditis 
  3. Postpartum thyroiditis ( temporary thyroiditis that may occur after childbirth)
  4. Iodine deficiency 


Low levels of TSH means that one has overactive thyroid. It can be due to

  1. Excessive iodine in the body
  2. Graves’ disease
  3. Excessive thyroid hormone medication
  4. Consumption of natural supplements that contains thyroid hormone
  5. Drugs such as dopamine, opioids,steroids, supplement biotin can falsely give lower TSH level.

Thyroid Disorder and Pregnancy

Thyroid disorder can make the pregnancy complicated, so TSH screening test can be recommended. Pregnant women who do not have symptoms of thyroid disorder can be screened for thyroid problems in the following conditions 

  1. Have family history of thyroid disease
  2. Have history of premature birth, miscarriage, or infertility 
  3. Obese significantly (body mass index over 40)
  4. Living in iodine deficient area
  5. Age is above 30 years
  6. Other risk factors are present


TSH blood test is a routine test like T3, T4 tests to know whether thyroid is functioning well or not. TSH blood level depends upon several factors as mentioned above. So, based upon your clinical case the normal range of TSH is determined. Usually TSH test is done with T3, T4 and other relevant tests and it is determined by your clinical features. So, consulting with your doctor and following the doctors instructions is important. If one is taking thyroid medications, do not stop the medication without consulting with the doctor. Normal functioning thyroid is vital for our body to live healthily. So, never hesitate to go for thyroid tests if your doctor prescribes to do so. Stay healthy and happy.


  1. Williams Textbook of Endocrinology E-Book
  2. Harrison’s Principles of Internal Medicine 20th edition
  3. Goldman-Cecil Medicine
  4. Guyton and Hall Textbook of Medical Physiology
  5. Davidson’s Principles and Practice of Medicine 23rd edition

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