What Are T3 & T4?

T3 and T4 are shortened references to two very important hormones that are produced by the thyroid gland to regulate the body's metabolism. If you have a thyroid disorder, you may have seen T3 and T4 scratched in your doctor's notes or on the lab sheet you present when you get your blood drawn. You might want to take a closer look at what these hormones are and why they are important.

The Thyroid Hormone

The thyroid is a small gland at the front of your neck. By taking in iodine that's a part of your regular diet, the gland produces what is known as the thyroid hormone. While this is often used in singular tense, the thyroid hormone is actually comprised of two separate hormones: T3 and T4.


Thyroxine is called T4 because it contains four iodine atoms. It presents in the bloodstream in two forms--that which attaches itself to proteins that prevent it from entering parts of the body that don't require thyroid hormone; and "free T4" (FT4), which enters targeted tissues directly to assist metabolic functions.


T3 is a shortened name for triiodothyronine. T3 is actually T4 that is converted into a more useful form in the liver and other tissues, such as the brain. During this process, it loses one of its iodine atoms, which is why it's called T3.

TSH and T4

The amount of T4 found in the body is dependent on a thyroid stimulating hormone (TSH) that is released by the pituitary gland in the brain. If the pituitary gland sees that there's not enough T4 circulating in the blood stream, it dispatches TSH to the thyroid gland to encourage it to work harder.

T3 & T4 Levels

Someone with a healthy metabolism with have levels of T3 and T4 within the range of normal. However, below normal levels of T4 and high levels of TSH can signal that a patient suffers from a low-functioning thyroid. Similarly, low levels of TSH and high levels of T3 may indicate that a patient may have an over-active thyroid.

About the Author

Lisa Sefcik has been writing professionally since 1987. Her subject matter includes pet care, travel, consumer reviews, classical music and entertainment. She's worked as a policy analyst, news reporter and freelance writer/columnist for Cox Publications and numerous national print publications. Sefcik holds a paralegal certification as well as degrees in journalism and piano performance from the University of Texas at Austin.