Both T3 and T4 are secreted by the thyroid gland, the hormones are stored in follicle stems between thyroid cells.
Normal Thyroid cells. (Place mouse over to see a thyroid gland.)
The thyroid gland is an endocrine gland at the front of the neck, just below the Adams apple; it is made up of two lobes lying along the trachea joined by a thin band of thyroid tissue called the isthmus.
The thyroid gland is controlled by the pituitary gland, situated at the base of the brain. The pituitary gland stimulates the production of T3 and T4 by producing Thyroxine Stimulating Hormone (TSH), thyrotropin when the levels of thyroid hormones are low. Once T3 and T4 levels are high enough TSH levels decrease and the production of thyroid hormones decreases.
The Pituitary gland is regulated by the hypothalamus which is part of the brain and secretes TSH Releasing Hormone, TRH.