The thyroid is a small gland at the front of your neck. It takes in iodine that is part of your regular diet, and produces what is known as the thyroid hormone. The thyroid hormone is 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.
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