01. Overview of Endocrinology

• hormone = substance secreted directly into the blood by specialised cells, present in small concentrations
• endocrine glands:

  • hypothalamus, pituitary, thyroid, parathyroids, adrenals, pancreas, ovary and testes
• three super families of hormones:

  1. peptide hormones
  2. amino acid derivatives
  3. steroid hormones and small molecules
• peptide hormones are water soluble, and can be stored in vesicles/granules and released by exocytosis in response to a signal

  • stored as they take a while to make but can act quickly giving rapid response
  • e.g. ACTH and insulin
  • often preprohormones
• preprohormones and prohormones are inactive until the inactive segment is removed, usually by an enzyme around the target tissue
• amino acid derivatives are thyroid hormones synthesised from tyrosine in the thyroid

  • stored until needed
  • includes catecholamines, the fastest acting hormones
• T4 contains 4 iodine residues but is metabolised to T3 which is more active

  • only known need for iodine in diet
• steroid hormones are lipophilic and produced on demand, not stored

  • mostly produced by adrenals
  • made from cholesterol
  • e.g. glucocorticoids, androgens, thyroid hormone and vitamin D
  • slowest acting hormones
• StAR is the enzyme needed to turn cholesterol into pregnenolone in the mitochondria

  • rate limiting step in steroid hormone synthesis