Endocrinology

• Endocrine


  Release of chemical messengers from an organ into the bloodstream
• Exocrine


  Release of hormones into ducts
• There are 80 recognised human hormones
• Major endocrine organs

  • Hypothalamus
  • Pituitary
  • Thyroid
  • Parathyroid
  • Adrenal
  • Pancreas
  • Ovaries
  • Testes
• The heart releases the hormone atrial natriuretic peptide (ANP)
• Protein hormones need to bind to a receptor on cells because they are water-soluble and therefore are not able to get inside the phospholipid membrane
• What happens when a hormone binds to a receptor


  It triggers a biochemical cascade e.g. G protein activates adenyl cyclase
• Cellular events hormone binding affects


  • Opening/closing of ion channels
  • Activation/deactivation of enzymes
• Lipid soluble hormones


  • These molecules are able to go inside of the phospholipid membrane - receptors are inside the cell
  • They target DNA in the nucleus - this is why testosterone and oestrogen can affect how we look
• Hormone release is stopped

  Once the hormone induces the required effect
• How the target organ communicates with the organ releasing the hormone

  • Humoral: alters something in the circulation
  • Neural: via a nerve signal if the organ is connected to the endocrine organ
  • Hormonal: target organ makes its own hormone that's released into the circulation
• Synergy


  2 or more hormones have the same effect on the organ
• Antagonism


  One hormone opposes the action of another
• Hypothalamus


  Affects appetite, body temperature regulation
• The hypothalamus is the only part of the brain not fully encased by the blood-brain barrier, so it has access to systemic blood and can sense changes in hormone levels
• Pituitary gland


  Nicknamed 'Master Gland'
• The hypothalamus controls the pituitary gland
• The pituitary gland is connected to the hypothalamus by a stalk called the infundibulum
• The pituitary gland secretes 9 hormones
• Posterior pituitary


  • Secretes neurohormones that were made in the hypothalamus
  • Composed of neural tissue => cannot produce hormones
• Anterior pituitary


  Composed of glandular tissue, produce/secrete many hormones such as GH, TSH, FSH, LH
• Tropic hormones


  Hormones that stimulate other glands to release their hormones e.g. thyroid stimulating hormone targets the thyroid, causing the release of thyroid hormone
• Gonadotropins


  FSH and LH
• Growth hormone


  • Hormone secreted by anterior pituitary gland that stimulates most cells to increase in size and number
  • Main target is bone and muscle
  • Increases bone length and muscle mass
• Metabolic effect of growth hormone

  • Direct and indirect
  • Increases fat usage for fuel
  • Promote growth
• Hypersecretion of growth hormone can cause acromegaly - people don't stop growing - can reach seven feet tall
• Hormones involved in the regulation of growth hormone

  • GHRH - growth hormone releasing hormone
  • GHIR - growth hormone inhibiting hormone
• Two primary posterior hormones

  • Oxytocin
  • Antidiuretic hormone (ADH)
• Oxytocin


  • Stimulates uterine contractions during childbirth (triggered by the stretch of the uterine wall)
  • Stimulates release of milk (triggered by suckling) (prolactin causes production of milk, oxytocin causes the release)
• Antidiuretic hormone (ADH)


  • Inhibits urine production; causes kidneys to reabsorb water; increases blood pressure
  • Stimuli for release: pain, low blood pressure
  • Inhibitors: alcohol, diuretic drugs
• ADH deficiency causes diabetes insipidus - huge output of urine and intense thirst
• ADH deficiency can be caused by damage to the CNS - hypothalamus or pituitary
• What the thyroid gland produces

  • Thyroxine
  • Calcitonin
• Thyroxine


  • Also called thyroid hormone
  • Vital for life due to its role in metabolism
  • Totally under the control of the hypothalamus-pituitary axis
• Calcitonin is not controlled by the hypothalamus-pituitary axis
• Structure of thyroid gland

  • Made up of follicles
  • Hormones are stored inside - colloid - gelatinous mix of hormone and iodine (iodine deficiency can interfere with function)
• Iodine is a vital mineral for normal thyroid function
• The thyroid is an endocrine gland - the colloid is secreted directly from the follicles into the circulation
• Two types of thyroid hormones

  • T4 - thyroxine
  • T3 - triiodothyronine
• Adult brain, testes, uterus and thyroid do not need thyroid hormone