The thyroid gland lies in the front of the throat, and secrets a combination of two hormones; about one-third T3 (triiodothyronine), which is the active thyroid hormone and about two-third T4 (thyroxine), which is a storage hormone. T4 can be turned into T3 in the tissues, mainly the liver, making the liver able to regulate the thyroid function.
The active thyroid hormone T3 controls metabolism and cellular energy production. Low cellular energy production makes the body unable to overcome the demands placed on it from the environment. In the low cellular energy state, the body loses structure and control and lets harmful degenerative processes occur.  Thus low thyroid function has devastating consequences for the body, creating many different symptoms as the body's tissues are not functioning optimally.