A hormone is a chemical messenger that signals to specific cells to change their behavior in specific ways. Hormones are secreted into the bloodstream by ductless endocrine glands, which form part of the endocrine system. Hormones may be classified into three broad categories:
- Amines: Hormones which are derived from the amino acid tyrosine (Dopamine, epinephrine, norepinephrine, thyroid hormones).
- Peptides: Hormones which are generated by protein synthesis and stored for release (Pituitary gland hormones).
- Steroids: Hormones which are derived from cholesterol (Estrogen, Testosterone, Aldosterone, Cortisol).
The following are a few significant examples:
- Estrogen from the ovaries, which induces development of secondary female characteristics during puberty and development of uterine lining following menstruation.
- Testosterone (secreted from testes) induces secondary male characteristics during puberty.
- Adrenaline from the adrenal glands prepares body for physical activity (often known as the "fight or flight" hormone).
- ADH (Antidiuretic hormone) from the pituitary gland, which makes the walls of collecting ducts in kidneys more permeable to water, which in turn increases extracellular volume.
- Thymulin (also called Thymic Factor or its old name Facteur Thymique Serique) affects T-cells
- Wile, Dr. Jay L. Exploring Creation With General Science. Anderson: Apologia Educational Ministries, Inc. 2000