The thyroid gland secretes two primary hormones into the bloodstream: T3 (triiodothyronine) and T4 (thyroxine). While thyroxine (T4) is directly secreted by the thyroid gland, a significant portion of the T3 hormone in circulation is produced when the body converts T4 into the more active T3. This conversion occurs because T3 is the biologically more active form. The liver and kidneys primarily undertake the task of converting inactive T4 hormone into active T3.

T3 and T4 hormones play a vital role in maintaining overall health by regulating numerous critical functions in the body. These functions include:

* Regulation of metabolic rate
* Control of heart and digestive system functions
* Maintenance of muscle function
* Brain development and functions
* Support for bone health