What is Thyroid Stimulating Hormone (TSH) test?

Thyroid stimulating hormone (TSH) is secreted by the anterior pituitary gland. It is responsible for regulating the levels of thyroid hormones (T3 and T4). Thyroid hormones are secreted from a butterfly-shaped gland in the neck, and these hormones (specifically T4) play a vital role in various important metabolic functions and the growth and development of body.

TSH is considered to be the best laboratory test available for the diagnosis of thyroid-related diseases. Results of this test are usually combined with those of T3 and T4 for a comprehensive diagnosis. As the levels of thyroid hormone increase in blood, the brain orders pituitary gland to secrete less TSH. So, by comparing the amounts of TSH with T3 and T4, it becomes easier to diagnose an underactive or overactive thyroid gland.

  1. Why is TSH test performed?
  2. How do you prepare for TSH test?
  3. How is TSH test performed?
  4. TSH test results and normal range

Improper functioning of thyroid gland due to various causes may lead to an imbalance in thyroid hormone levels resulting in conditions such as hyperthyroidism or hypothyroidism.

Common symptoms of hyperthyroidism include:

  • Anxiety
  • Loss of weight
  • Fatigue
  • Muscle weakness
  • Intolerance to heat
  • Heart palpitations
  • Irregular and infrequent menses in women
  • Tremors
  • Disturbances in sleep

Symptoms of hypothyroidism include:

Pregnancy can also affect thyroid functions. Thyroid disease during pregnancy may have a detrimental effect on the mother or the foetus. Therefore, it becomes essential to test thyroid hormone status in a pregnant woman, especially if she is experiencing any of these symptoms.

A TSH test in combination with other investigations plays a vital role in the diagnosis and proper treatment of thyroid disorders (hyper- or hypothyroidism). This test is also used to monitor the treatment of thyroid diseases.

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No special preparations such as fasting are required for TSH. Sometimes, a  doctor may suggest additional tests other than TSH that may require fasting. Therefore, it is important to talk to your doctor about any special precautions that may need to be taken before you go for the tests.

Some medications may affect the results of this test; therefore, it is essential to inform the doctor about any ongoing medications (either prescribed or over-the-counter). As thyroid disorders are common during pregnancy, make sure that your doctor knows if you are pregnant or suspect pregnancy.

  • Medications such as heparin therapy, corticosteroids and aspirin lower TSH levels.
  • TSH injections and the intake of lithium, iodide and potassium increase TSH levels.

TSH is a simple blood test that can be performed without fasting.

  • A nurse or laboratory technician will first swab your skin (generally your arm) with alcohol or an antiseptic and then take a blood sample by inserting a needle into a vein. A small amount of blood is usually required to perform this test. Slight pain may be felt at the site of insertion of the needle but the test otherwise is painless.
  • The collected blood is then transferred to a test tube. The rest of the procedure may require approximately 5-10 minutes.

If an individual experiences prolonged pain or bleeding from the site of blood withdrawal, it is important to inform the doctor to check for any reaction.

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TSH test measures blood levels of the thyroid-stimulating hormone in milliunits per litre (mU/L).

Normal results:

Normal values of TSH in the blood range from 0.4-5.5 mU/L for an adult (the normal range may vary slightly in different laboratories).

Abnormal results:

TSH levels above the normal range mean the thyroid gland is underactive. Low thyroid hormone levels cause stimulation of the anterior pituitary gland, leading to the secretion of more TSH. Some causes of hypothyroidism are:

  • Thyroiditis i.e., inflammation of the thyroid gland
  • Hashimoto’s thyroiditis. i.e., An autoimmune disorder
  • Congenital hypothyroidism, i.e., hypothyroidism present since birth
  • Iodine deficiency
  • Surgery for partial or full removal of the thyroid gland
  • Some medications

TSH levels below the normal range mean the thyroid gland is overactive, producing more thyroid hormones. High levels of thyroid hormones cause suppression of the anterior pituitary gland, leading to secretion of less TSH. Some of the causes of hyperthyroidism are.

  • Graves’ disease. i.e., an overactive thyroid gland
  • Overactive thyroid nodules within the thyroid gland
  • Overdose of medicines for hypothyroidism
  • Tumour of the pituitary gland
  • Thyroiditis
  • Excessive iodine consumption

As seen earlier, the interpretation of TSH test depends on various factors like pregnancy, thyroid disorders, medications, etc. Sometimes, based on the results of the TSH test, a doctor may advise further investigations like other hormonal tests, radiological examinations such as computed tomography (CT) or magnetic resonance imaging (MRI) to confirm the exact cause of thyroid disease. Based on all these findings, he/she will prescribe medications to treat the condition and may ask for regular monitoring of TSH levels.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. This information is purely from an educational point of view and is in no way a substitute for medical advice by a qualified doctor. 


  1. Pirahanchi Y, Jialal I. Physiology, Thyroid Stimulating Hormone (TSH) [Updated 2018 Oct 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan
  2. Armstrong M, Fingeret A. Physiology, Thyroid Function. [Updated 2019 Feb 6]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan
  3. Sheehan MT. Biochemical Testing of the Thyroid: TSH is the Best and, Oftentimes, Only Test Needed - A Review for Primary Care. Clin Med Res. 2016;14(2):83–92.
  4. Sharma M, Aronow WS, Patel L, Gandhi K, Desai H. Hyperthyroidism Med Sci Monit. 2011;17(4):RA85–RA91.
  5. National Institute of Diabetes and Digestive and Kidney Diseases [internet]: US Department of Health and Human Services; Hypothyroidism (Underactive Thyroid)
  6. El-Shafie KT. Clinical presentation of hypothyroidism. J Family Community Med. 2003;10(1):55–58.
  7. Alemu A, Terefe B, Abebe M, Biadgo B. Thyroid hormone dysfunction during pregnancy: A review. Int J Reprod Biomed (Yazd). 2016;14(11):677–686.
  8. National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; What Are the Risks of Blood Tests?; [updated 2012 Jan 6; cited 2017 Mar 15]
  9. University of Rochester Medical Center [Internet]. Rochester (NY): University of Rochester Medical Center; c2017. Health Encyclopedia: Thyroid Stimulating Hormone; [cited 2017 Mar 15]
  10. Erik, A. et. al. (2017 March 1). 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum . THYROID Volume 27, Number 3, 2017

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