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Thyroid Stimulating Hormone (TSH) Blood Test

Thyroid Stimulating Hormone (TSH) Blood Test

A TSH blood test is often done to diagnose thyroid problems through evaluation of the function of the thyroid gland, which is controlled by the pituitary gland that produces the thyroid stimulating hormone (TSH).
Sonia Nair
Last Updated: Jun 3, 2018
An early morning blood test is preferred, as the TSH level varies throughout the day.
The thyroid is the butterfly-shaped gland located in front of the windpipe, on the anterior side of the neck, just below the thyroid cartilage. This gland produces hormones, mainly thyroxine (T4) and triiodothyronine (T3). Calcitonin is another hormone produced by the thyroid gland. This hormone helps in maintaining the required level of calcium in the body. The main function of the thyroid gland is to maintain the rate of metabolism and growth. It can also affect the functioning of other bodily systems. In short, the hormones produced by the thyroid gland play a major role in the day-to-day functions of the body.
The thyroid gland is controlled by the hypothalamus in the brain, and the pituitary gland that is located at the base of the brain. The hypothalamus releases thyrotropin-releasing hormone (TRH), which induces the pituitary gland to secrete thyroid stimulating hormone (TSH). TSH regulates the hormone production of the thyroid gland. A low level of T3 and T4 in the blood stimulates the pituitary to produce more TSH and vice versa. Hence, evaluating the level of TSH in the blood helps in assessing the functioning of the thyroid gland.
Why to do a TSH Test
The most common problems related to the functioning of the thyroid gland are hyperthyroidism and hypothyroidism. Hyperthyroidism denotes an overactive thyroid gland characterized by an excess of thyroid hormones in the blood. Hypothyroidism refers to an underactive thyroid gland and a low level of thyroid hormones. A person suffering from hyperthyroidism may experience symptoms, like rapid heartbeat, weight loss, nervousness, irregular menstrual periods, diarrhea, puffy eyes, insomnia, etc. Hypothyroidism is characterized by symptoms, like irregular menstrual periods, weight gain, fatigue, hair loss, constipation, cold intolerance, and dry skin.
The TSH blood test is suggested for those people who experience the symptoms of hyperthyroidism or hypothyroidism. Those with an enlarged thyroid gland may also be required to do this test, which is also useful for monitoring the results in those who are already undergoing treatment for thyroid problems. This test is also a part of the newborn screening program in many countries, including the United States. Some medical organizations suggest this test for women above the age of 35, whereas some others recommend it for women above 65 and men above 70. This test is also recommended for pregnant women, and for those who are planning to get pregnant.
TSH Blood Test - Preparation and Procedure
This is a simple blood test that is not associated with any risk. Usually, the blood is drawn from the vein in the arm. Some medicines may interfere with the test results. They include lithium, dopamine, prednisone, etc. So you must inform your health care provider about your medical conditions and current medication. Stop the medicines temporarily, only if the doctor asks you to do so. Certain tests that use radioactive materials may also interfere with the results of a TSH blood test. Even X-rays that use iodine dye may affect a TSH test outcome. This happens when such tests are done within 4 to 6 weeks immediately preceding the date of TSH test. Even extreme stress may affect the test results. Pregnant women have a low TSH level, especially during the first three months of pregnancy.
Interpretation of Results
Normal Range: According to the National Institute of Health (NIH), the normal TSH range is 0.4 to 4.0 milli-international units per liter (mIU/L). If the TSH level is below 0.4 mIU/L, the person has an overactive thyroid that indicates hyperthyroidism. This condition could be due to Grave's disease, thyroiditis, or thyroid nodules. If the level is above 4.0 mIU/L, the thyroid is underactive, and the person has hypothyroidism. This could be due to any abnormality of the pituitary gland or Hashimoto's disease. If the TSH is low, the T3 an T4 levels have to be high, and vice versa. If both TSH and thyroid hormones are either low or high, then the problem could be primarily related to the pituitary gland.
Age Group: In case of normal infants, the TSH range is between 1.1 to 17 mIU/L during the first three days after birth. The values reduce to 0.6 to 10 mIU/L at 10 weeks. After 14 months, the normal TSH level is around 0.4 to 7.0 mIU/L. The normal TSH level for kids in the age group of 5 to 14 years ranges between 0.4 to 5 mIU/L. Above 14 years, the TSH range is 0.4 to 4.0 mIU/L.
Pregnancy: In case of pregnant women, the normal TSH level range is 0.1 to 2.5 mIU/L during the first trimester, 0.2 to 3.0 mIU/L during the second trimester, and 0.3 to 3.0 mIU/L during the third trimester.
However, health experts suggest a new reference range. According to the American Association of Clinical Endocrinologists, the normal range of TSH should be between 0.3 to 3.0 mIU/L. While some health care providers follow the new values, others stick to the old reference range. So the results are often interpreted, along with the T3 and T4 test results.
Disclaimer: This article is for informational purposes only, and should not be used as a replacement for expert medical advice.