1. Thyroid hormones are derivatives of the the amino acid tyrosine bound covalently to iodine. The two principal thyroid hormones are: Thyroxine (also known as T4 or L-3,5,3',5'- tetraiodothyronine) Triiodothyronine (T3 or L-3,5,3'-triiodothyronine) The thyroid hormones are basically two tyrosines linked together with the critical addition of iodine at three or four positions on the aromatic rings. The number and position of the iodines is important. Thyroid hormones

2. Thyroid peroxidase, an integral membrane protein present in the apical (colloid-facing) plasma membrane of thyroid epithelial cells. Thyroid peroxidase catalyzes two sequential reactions: 1.Iodination of tyrosines on thyroglobulin (also known as "organification of iodide"). 2.Synthesis of thyroxine or triiodothyronine from two iodotyrosines.

3. Physiologic Effects of Thyroid Hormones It is likely that all cells in the body are targets for thyroid hormones. Thyroid hormones have profound effects on many physiologic processes, such as 1.Development 2.Growth and 3.Metabolism 4.Other effects a.CVS b.CNS c.RS

4. Thyroid-releasing hormone (TRH) from the hypothalamus stimulates TSH from the pituitary, which stimulates thyroid hormone release. As blood concentrations of thyroid hormones increase, they inhibit both TSH and TRH, leading to "shutdown" of thyroid epithelial cells. Later, when blood levels of thyroid hormone have decayed, the negative feedback signal fades, and the system wakes up again. Control of Thyroid Hormone Synthesis and Secretion

5. Growth Hormone (Somatotropin) Growth hormone is a protein hormone of about 190 amino acids that is synthesized and secreted by cells called somatotrophs in the anterior pituitary. It is a major participant in control of several complex physiologic processes, including growth and metabolism. Growth hormone is also of considerable interest as a drug used in both humans and animals.

6. Physiologic Effects of Growth Hormone A critical concept in understanding growth hormone activity is that it has two distinct types of effects: 1.Direct effects 2.Indirect effects Two major roles of growth hormone are- 1.Effects on Growth 2.Metabolic Effects a.Protein metabolism b.Fat metabolism c.Carbohydrate metabolism

7. Control of Growth Hormone Secretion Production of growth hormone is modulated by many factors, including stress, exercise, nutrition, sleep and growth hormone itself. However, its primary controllers are two hypothalamic hormones and one hormone from the stomach: