The thyroid is a very important gland that is part of the endocrine system. It plays a major role in regulating the metabolism of all cells in the body. The pituitary gland secretes TSH (thyroid stimulating hormone) signaling the thyroid to secrete T4 (and some T3). T4 is not the active thyroid hormone. T4 must be converted to T3. It is T3 that is biologically active and influences the activity of all cells and tissues of the body.