The Thyroid Gland
The Thyroid gland is a hormone secreting gland located in the neck just inferior to cricoid cartiallage/ hyoid bone and anterior to the trachea. It is responsible for the production of numerous thyroid hormones that play an important role in metabolic processes in the body. It is supplied by thyroid artery branches of the aorta which is located inferior to the gland.
The Thyroid gland produces many hormones but the two most important when considering Hyper and Hypo Thyroidism is Thyroxine (T4) and Triiodothyronine (T3); these hormones are responsible for increasing metabolic rate in the tissues they act on. They also have an imporant role to play in cardiovascular function and development. T3 is considered to be more potent than T4, owing to its ability to bind more actively to tissue receptors. T4 is converted to T3 in target tissues, it has a shorter half life than T4.
Negative feedback system –
Stimulation of the thyroid gland to produce T3 and T4, occurs as a result of hormones secreted by the pituitary gland and hypothalamus. Thyrotropin Releasing Hormone (TRH) is released in a systematic manner by the hypothalamus, which in turn causes the pituitary gland to release Thyroid Stimulating Hormone (TSH). It is TSH which binds to receptors on the thyroid gland and stimulates the release of T3 and T4. This works by way of a negative feedback system, with TSH secretion increasing or decreasing production in response to decreases or increases of T3/T4 respectively.