TSH Test – All You Need to Know

Published on August 13, 2022

The thyroid gland secretes hormones that are vital for various body functions.  Thyroid dysfunction is usually detected through blood tests.  If an abnormality is detected, further tests may be carried out to find out the exact cause of the problem.

Thyroid-stimulating Hormone (TSH)

A common hormone that is measured from a blood sample to assess the thyroid function is the thyroid-stimulating hormone (TSH). TSH is not secreted by the thyroid gland but by a small gland at the base of the brain called the pituitary. Once it is released into the blood, it sends signals to the thyroid to secrete the thyroid hormones, called T4 and T3. The released thyroid hormones in turn control the levels of the TSH, by sending signals to the pituitary.

Thyroid-stimulating Hormone (TSH) – Interpretation of Normal and Abnormal

The normal range of TSH in the blood is between 0.4 to 4 milli-international units per liter. The exact levels may vary with the testing laboratory. Fasting is not necessary before taking the blood sample to estimate the TSH level.

If the level of TSH is lower than normal, it indicates that the levels of thyroid hormones are high in the blood, that is, the patient is hyperthyroid.

If the level of TSH is higher than normal, it indicates that the levels of thyroid hormones are low in the blood, that is, the patient is hypothyroid.

The levels of TSH may also be abnormal in diseases of the pituitary, in which case, the levels of other hormones secreted by the pituitary will also be abnormal.

Thyroid-stimulating Hormone (TSH) Test – Reliability

The TSH test is usually a reliable test in otherwise normal individuals. It may be used alone as an initial screening test for thyroid function. However, it may not correctly indicate the presence of a thyroid dysfunction in certain conditions such as early pregnancy, in patients with severe illness or those taking high doses of corticosteroid medications.


Pirahanchi Y, Toro F, Jialal I. Physiology, Thyroid Stimulating Hormone. [Updated 2022 May 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK499850/ Available under Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/)

Salman Razvi, Sindeep Bhana, Sanaa Mrabeti, “Challenges in Interpreting Thyroid Stimulating Hormone Results in the Diagnosis of Thyroid Dysfunction”, Journal of Thyroid Research, vol. 2019, Article ID 4106816, 8 pages, 2019. https://doi.org/10.1155/2019/4106816 Available under Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/)

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