The thyroid hormone process begins when the pituitary gland in the brain produces thyroid-stimulating hormone (TSH). TSH then acts upon the thyroid gland to produce thyroid hormones. The two main thyroid hormones produced are thyroxine (T4) and triiodothyronine (T3). Although the thyroid gland produces more T4 (80 percent) compared with T3 (20 percent), T3 is 300 percent more active than T4 and is the thyroid hormone responsible for increasing metabolism. In fact, much of the T4 is converted into the more active T3 inside the cells of the body.