Blood Tests

If you are suspected of having hypothyroidism, your doctor may order blood tests that will measure the levels of your thyroid hormones. Below is a list of commonly used tests to diagnose hypothyroidism or thyroid conditions.

Free Thyroxine (T4) - FT4 Test
This test measures the levels of thyroxine (T4) in your blood since a low level could indicate hypothyroidism.

Free Triiodothyronine (T3) – FT3 Test
This test measures the levels of triiodothyronine (T3) in your blood since a low level could also indicate hypothyroidism.

The Free T3 and Free T4 tests measure the T3 and T4 that are carried in the blood and unattached to proteins. Most of the T4 and T3 in the body are attached to carrier proteins and not free-flowing. Since certain conditions such as pregnancy and liver disease can increase carrier protein concentrations, doctors will get a more accurate reading by assessing the levels of free T3 and T4.

Thyroid Antibody Test
The immune system produces antibodies to protect us from foreign invaders. These antibodies are produced by the white blood cells (lymphocytes) to destroy harmful attackers such as bacteria or viruses. People who suffer from thyroid disease may produce antibodies (thyroid peroxidase and thyroglobulin) that also attack the body, specifically the thyroid gland. Positive levels of these antibodies found in the blood can indicate conditions such as Hashimoto’s thyroiditis or autoimmune thyroid disease.

TSH Test
In the early stages of hypothyroidism, your thyroid hormone levels may still be in the normal range. Therefore, doctors often rely on the TSH (thyroid-stimulating hormone) blood test for diagnosing hypothyroidism.

Understanding How Your Body Functions
What is TSH?
TSH stands for “thyroid-stimulating hormone” and its name describes its function. It is a hormone produced by the pituitary to stimulate the thyroid to produce more thyroid hormones if needed.

The Purpose of TSH
Your body is a highly complex messaging system in which your cells are constantly communicating to keep your hormone levels at a stable, normal range. When you are suffering from hypothyroidism, your thyroid gland is not producing adequate amounts of thyroxine (T4). When thyroxine (T4) levels fall below normal, your pituitary secretes more TSH which communicates to the thyroid to produce more thyroxine (T4). In contrast, if your thyroxine (T4) level increases to above normal, your pituitary will “back off” and cease secreting TSH.

Since during hypothyroidism the thyroid is not capable of producing more thyroxine (T4), the pituitary continues to secrete more TSH, which results in elevated TSH levels.

TSH blood tests are commonly used to diagnose hypothyroidism since a high TSH level indicates a low thyroxine (T4) level. Additionally, elevated TSH levels could also signal other problems with the pituitary gland. Your doctor may perform additional testing to rule out other issues. Your doctor will discuss the results of your test, make a diagnosis and begin the proper course of treatment, for you.

Patient Education - Endocrine Encyclopedia. (n.d.). UCLA Endocrine Surgery. Retrieved October 12, 2011 from

American Thyroid Association. (2005). Thyroid Function Tests [Brochure]. Retrieved from