Hormones and the Endocrine System

Hormones

• A hormone is a chemical substance produced in the body that has a specific regulatory effect on the activity of an organ or organs.
• Hormones are produced at one site in the body and exert their action(s) at distant sites through the endocrine system.
• Some hormones exert their action locally through the paracrine system, while others exert their action on the cells of origin, regulating their own synthesis and secretion via an autocrine system.

Classification

• Hormones are classified into three categories:
  • Polypeptide or protein hormones (e.g., ACTH, insulin, parathyroid hormone)
  • Steroid hormones (e.g., adrenal cortical hormones, androgens, estrogens)
  • Derivatives of amino acids (e.g., thyroxine, catecholamines)

Polypeptide or Protein Hormones

• These hormones are generally water-soluble and circulate freely in plasma as whole molecules or as active or inactive fragments.
• They have a short half-life of ≤10 to 30 minutes in plasma, and their concentration may fluctuate in physiologic and pathologic circumstances.
• They initiate their response by binding to cell membrane receptors and exciting a "second messenger" system.

Steroid Hormones

• These hormones are hydrophobic and insoluble in water.
• They circulate in plasma, reversibly bound to transport proteins, with only a small fraction free and unbound, available to exert physiologic action.
• They have a half-life of 30 to 90 minutes in plasma.
• Free steroid hormones enter the cell by passive diffusion and bind to intracellular receptors in the cytoplasm or nucleus.

Amino Acid-Related Hormones

• These hormones are derived from amino acids and are water-soluble.
• They circulate in plasma bound to proteins (e.g., thyroxine) or free (e.g., catecholamines).
• They interact with membrane-associated receptors and use a second messenger system.

Release and Action of Hormones

• Hormones have physiologic functions that can be broadly categorized into three areas:
  • Affecting growth and development
  • Exerting homeostatic control of metabolic pathways
  • Regulating the production, use, and storage of energy

Growth and Development

• Normal growth and development of the human organism is dependent on the complex integrative function of many hormones.
• Several pituitary hormones are responsible for the growth and development of endocrine glands themselves.

Homeostatic Control of Metabolic Pathways

• Hormones are involved in the homeostatic control of diverse and complex metabolic pathways.
• Insulin and glucagon are the two hormones primarily responsible for maintaining homeostasis of blood glucose levels.