What Is the Difference Between T4 & T3?

Every cell in the human body depends upon thyroid hormones to regulate metabolism. The thyroid gland takes the iodine found in many foods and converts it into thyroid hormones known as T3 and T4.


The thyroid hormones created from iodine are thyroxine, also known as (T4) and triiodothyronine, also known as (T3).

How They Work

The only cells in the body that can absorb iodine are the thyroid cells. These cells combine the iodine and an amino acid called tyrosine to make T3 and T4. The T3 and T4 are then released into the bloodstream and transported through the entire body to regulate metabolism.

The Difference

T3 is an active thyroid hormone 2. T4 is actually produced by your thyroid gland and is then converted by a deiodinase enzyme into the T3 hormone while in the liver and sometimes in other organs in lesser amounts. On its own, T4 is of little use. It is the T3 hormone that is the active hormone. The T3 hormone is up to five times more potent than the T4 hormone, and without having T4 converted into T3, one’s metabolic rate does not increase.