Hormones are chemical messengers that regulate various bodily functions. They come in three main types: lipid-derived, amino acid-derived, and peptide hormones. Each type has unique structural characteristics and mechanisms of action in target cells.

Understanding hormone types is crucial for grasping how the endocrine system works. These molecules control processes like metabolism, growth, and reproduction. Their diverse structures and functions showcase the complexity of cellular communication in the body.

Types of Hormones

Types of hormones by structure

• Lipid-derived hormones
  • Synthesized from cholesterol
  • Steroid hormones have four interconnected rings of carbon atoms (cortisol, testosterone, estrogen)
  • Eicosanoids are derived from fatty acids (prostaglandins, thromboxanes, leukotrienes)
• Amino acid-derived hormones
  • Synthesized from tyrosine or tryptophan amino acids
  • Include epinephrine, norepinephrine, and thyroid hormones (T3, T4)
• Peptide hormones
  • Composed of chains of amino acids linked by peptide bonds
  • Oligopeptides contain 2-50 amino acids (antidiuretic hormone ADH)
  • Polypeptides contain 50-200 amino acids (growth hormone GH)
  • Protein hormones have more than 200 amino acids (insulin)

Mechanisms of hormone action

• Lipid-derived hormones
  • Hydrophobic nature allows passage through cell membranes
  • Bind to intracellular receptors in the cytoplasm or nucleus
  • Hormone-receptor complex acts as a transcription factor regulating gene expression
  • Produce slower, long-lasting effects on target cells
• Amino acid-derived hormones
  • Thyroid hormones are lipophilic and pass through cell membranes
    • Bind to intracellular receptors and regulate gene expression
  • Catecholamines (epinephrine, norepinephrine) bind to cell surface receptors
    • Activate second messenger systems like cAMP for rapid effects
• Peptide hormones
  • Hydrophilic nature prevents passage through cell membranes
  • Bind to specific cell surface receptors on target cells
  • Activate second messenger systems (cAMP, IP3/DAG) for rapid effects
  • Produce faster, short-term effects compared to lipid-derived hormones
  • Initiate signal transduction pathways upon binding to hormone receptors

Examples and functions of hormones

• Lipid-derived hormones
  • Cortisol is a glucocorticoid produced by the adrenal cortex
    • Regulates metabolism, inflammation, and stress response
  • Testosterone is an androgen produced by the testes and adrenal cortex
    • Promotes male secondary sexual characteristics and muscle growth
  • Estradiol is an estrogen produced by the ovaries
    • Regulates female reproductive system and secondary sexual characteristics
• Amino acid-derived hormones
  • Thyroxine (T4) and triiodothyronine (T3) are produced by the thyroid gland
    • Regulate metabolism, growth, and development
  • Epinephrine and norepinephrine are produced by the adrenal medulla
    • Regulate the "fight or flight" response and cardiovascular function
• Peptide hormones
  • Insulin is produced by the pancreatic beta cells
    • Regulates blood glucose levels by promoting glucose uptake and storage
  • Growth hormone (GH) is produced by the anterior pituitary gland
    • Stimulates growth, cell reproduction, and cell regeneration
  • Antidiuretic hormone (ADH) is produced by the hypothalamus
    • Stored in the posterior pituitary gland
    • Regulates water balance by promoting water reabsorption in the kidneys

Endocrine System Regulation

• Endocrine glands secrete hormones directly into the bloodstream
• Negative feedback mechanisms maintain hormone levels within a normal range
  • Rising hormone levels inhibit further hormone production
• Positive feedback mechanisms amplify hormone responses in certain situations
  • Example: oxytocin release during childbirth