TSH is a hormone that is produced by the pituitary gland, located at the base of your brain. Its primary function is to regulate the production of thyroid hormones which regulate your metabolism, heart, muscles, brain development, and bone maintenance. This initial test will show you what the overall levels of your thyroid are.

Triiodothyronine (T3) is a thyroid hormone that is produced by the thyroid gland which helps maintain muscle control, brain function and development, heart, and digestive functions. It also plays an important role in the body’s metabolic rate as well as the maintenance of bone health. The T3 that doesn't bind to protein is called free T3 and circulates freely in your blood. Knowing your T3 levels will inform you of your overall thyroid health.

Thyroxine (T4) is the main hormone produced by the thyroid gland. This hormone plays a role in different body functions, including growth and metabolism. Some of your T4 exists as FT4, which means that it is not bonded to proteins in your blood, whilst T4 attaches to proteins. Knowing your T4 levels will help you understand your thyroid levels.

TPO is a protein produced by the thyroid gland. TPO is important for the conversion of T4 to T3. Autoimmune disorders stimulate an increased production of TPO antibodies which can disrupt normal protein functioning. Therefore, this test detects antibodies against TPO in the blood. If you have already been diagnosed with thyroid disease, your doctor may suggest a TPO antibody test together with other thyroid tests to help find the cause.

What is the thyroid? Why is it important?

The thyroid gland is a small butterfly-shaped endocrine gland in the neck, just in front of the windpipe (trachea). It is made up of two lobes — the right lobe and the left lobe, each about the size of a plum cut in half — and these two lobes are joined together. One of the thyroid’s main function is to produce hormones that are necessary for all the cells in your body to function normally. These hormones are called triiodothyronine (T3) and thyroxine (T4). Of importance, thyroid disorders are very common and tend mainly to occur in women, although anybody — men, teenagers, children, and babies — can be affected. Thyroid disorders may be either temporary or permanent.

Are there any medications that interfere with thyroid function testing?

Yes. There are many medications that may affect thyroid status. One important example is Biotin, which is a commonly taken over-the-counter supplement, and can cause thyroid function tests to appear to look abnormal when they are actually normal in the blood. Biotin should not be taken for two days before blood is drawn for thyroid function testing to avoid this effect.