A Simple Test Can Reveal a Lot
Most doctors rely on the TSH test for diagnosing hypothyroidism. It's a simple blood test that checks the level of thyroid-stimulating hormone (TSH) in the blood.
People with normal thyroid function typically have a TSH level that falls between 0.4 and 4.0 mIU/L. If you are diagnosed with hypothyroidism, the American Association of Clinical Endocrinologists recommends patients keep their TSH level between 0.45 and 4.12 mIU/L. Your doctor will determine the range that is right for you.
Understanding TSH and thyroid function
Your pituitary – a pea-sized gland located in the brain – produces TSH. In people with normal thyroid function, when thyroid hormone levels get too low, the pituitary makes more TSH to signal the thyroid to make more hormones. When the pituitary senses that enough hormones have been made, it backs off on its TSH production. This keeps everything in balance.
When you have hypothyroidism, the thyroid is not making enough of its main hormone, thyroxine. So the pituitary responds like it's supposed to and makes more TSH to signal the thyroid to make more thyroxine. But since the thyroid can't make more thyroxine, the pituitary doesn't know when to stop making TSH, and you end up with high TSH levels.
This is why the TSH test is commonly used – a high level of TSH usually indicates the person has a low thyroxine level, and this could mean hypothyroidism. A high TSH could also signal a problem with the pituitary gland or other parts of the brain. Your doctor may need to conduct additional tests to confirm hypothyroidism.
The FT4 test
Using the same sample of blood drawn for the TSH test, your doctor also may check your FT4 – or "free thyroxine" – level. This blood test measures the amount of thyroxine available for your body to use. A low FT4 level could mean hypothyroidism.
If your doctor diagnoses you with hypothyroidism, the next step will be discussing your treatment.
1. National Institute of Diabetes and Digestive and Kidney Diseases. National Endocrine and Metabolic Disease Information Service. Thyroid Function Tests. Available at: http://endocrine.niddk.nih.gov/pubs/thyroidtests/index.htm. Accessed March 1, 2013.
2. American Thyroid Association. Hypothyroidism. A booklet for patients and their families. http://www.thyroid.org/wp-content/uploads/patients/brochures/Hypothyroidism_web_booklet.pdf. Published 2013. Accessed January 23, 2014.
3. Garber JR, Cobin RH, Gharib H, et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Endocr Pract. 2012;18(6):988-1028.