Endocrine System

• Nervous and endocrine systems

  Major controlling systems of the body
• Nervous system

  Exerts rapid controls via nerve impulses
• Endocrine system

  Exerts more prolonged effects via hormones
• Hormonally regulated processes

  • Reproduction
  • Growth and development
  • Maintaining electrolyte, water, and nutrient balance
  • Regulating cellular metabolism and energy balance
  • Mobilizing body defenses
• Endocrine organs

  • Ductless, well-vascularized glands that release hormones directly into the blood or lymph
  • Small and widely separated in the body
• Purely endocrine organs

  • Pituitary
  • Thyroid
  • Parathyroid
  • Adrenal
  • Pineal glands
• Hypothalamus

  A neuroendocrine organ
• Other organs with endocrine tissue

  • Pancreas
  • Gonads
  • Placenta
• Local chemical messengers

  • Autocrines (act on the cells that secrete them)
  • Paracrines (act on a different cell type nearby)
• Hormones

  Most are steroids or amino acid based
• Steroid hormones

  Lipid soluble
• Amino acid-based hormones

  Water soluble, except for thyroid hormone
• Hormone action

  Stimulating or inhibiting characteristic cellular processes of their target cells
• Cell responses to hormone stimulation

  • Changes in membrane permeability
  • Enzyme synthesis, activation, or inhibition
  • Secretory activity
  • Mitosis
• Second-messenger mechanisms

  1. Hormone binds to plasma membrane receptor

  2. Couples to G protein

  3. G protein activates adenylate cyclase

  4. Adenylate cyclase catalyzes synthesis of cyclic AMP from ATP

  5. Cyclic AMP initiates reactions that activate protein kinases and other enzymes, leading to cellular responses
• Steroid hormones and thyroid hormone

  Enter their target cells and effect responses by activating DNA, which initiates messenger RNA formation leading to protein synthesis
• Stimuli that cause hormone release

  • Humoral
  • Neural
  • Hormonal
• Negative feedback

  Important in regulating hormone levels in the blood
• Nervous system


  Can in certain cases override or modulate hormonal effects
• Hormone receptors

  • Dynamic structures
  • High or low levels of stimulating hormones can change the number and/or sensitivity of hormone receptors
• Blood levels of hormones

  • Reflect a balance between secretion and degradation/excretion
  • Liver and kidneys are the major organs that degrade hormones
  • Breakdown products are excreted in urine and feces
• Hormone half-life and duration of activity


  Limited and vary from hormone to hormone
• Permissiveness

  One hormone must be present in order for another hormone to exert its full effects
• Synergism

  Two or more hormones produce the same effects in a target cell and their results together are amplified
• Antagonism

  A hormone opposes or reverses the effect of another hormone
• Pituitary gland

  Consists of a hormone-producing glandular portion (anterior pituitary or adenohypophysis) and a neural portion (posterior pituitary or neurohypophysis), which is an extension of the hypothalamus
• Hypothalamus

  1. Synthesizes two hormones that it exports to the posterior pituitary for storage and later release

  2. Regulates the hormonal output of the anterior pituitary via releasing and inhibiting hormones
• Posterior pituitary

  Stores and releases two hypothalamic hormones, oxytocin and antidiuretic hormone (ADH)
• Oxytocin

  Stimulates powerful uterine contractions, which trigger labor and delivery of an infant, and milk ejection in nursing women
• Antidiuretic hormone (ADH)

  • Stimulates the kidney tubules to reabsorb and conserve water, resulting in small volumes of highly concentrated urine and decreased plasma solute concentration
  • Released in response to high solute concentrations in the blood and inhibited by low solute concentrations in the blood
  • Hyposecretion results in diabetes insipidus
• Anterior pituitary hormones

  • Growth hormone (GH)
  • Thyroid-stimulating hormone (TSH)
  • Adrenocorticotropic hormone (ACTH)
  • Follicle-stimulating hormone (FSH)
  • Luteinizing hormone (LH)
  • Prolactin (PRL)
• Growth hormone (GH)

  • An anabolic hormone that stimulates growth of all body tissues but especially skeletal muscle and bone
  • May act directly, or indirectly, via insulin-like growth factors (IGFs)
  • Mobilizes fats, stimulates protein synthesis, and inhibits glucose uptake and metabolism
  • Secretion is regulated by growth hormone–releasing hormone (GHRH) and growth hormone–inhibiting hormone (GHIH), or somatostatin
  • Hypersecretion causes gigantism in children and acromegaly in adults
  • Hyposecretion in children causes pituitary dwarfism
• Thyroid-stimulating hormone (TSH)


  • Promotes normal development and activity of the thyroid gland
  • Thyrotropin-releasing hormone (TRH) stimulates release of TSH
  • Negative feedback of thyroid hormone inhibits it
• Adrenocorticotropic hormone (ACTH)

  • Stimulates the adrenal cortex to release corticosteroids
  • Corticotropin-releasing hormone (CRH) triggers ACTH release
  • Rising glucocorticoid levels inhibit it
• Gonadotropins (FSH and LH)

  • Regulate the functions of the gonads in both sexes
  • FSH stimulates sex cell production
  • LH stimulates gonadal hormone production
  • Gonadotropin levels rise in response to gonadotropin-releasing hormone (GnRH)
  • Negative feedback of gonadal hormones inhibits gonadotropin release
• Prolactin (PRL)

  • Promotes milk production in humans
  • Its secretion is inhibited by prolactin-inhibiting hormone (PIH)
• Thyroid gland


  • Located in the anterior neck
  • Thyroid follicles store colloid containing thyroglobulin, a glycoprotein from which thyroid hormone is derived
• Thyroid hormone (TH)

  • Includes thyroxine (T4) and triiodothyronine (T3)
  • Increase the rate of cellular metabolism
  • Consequently, oxygen use and heat production rise
• Secretion of thyroid hormone

  1. Prompted by TSH

  2. Requires the follicular cells to take up the stored colloid and split the hormones from the colloid for release

  3. Rising levels of thyroid hormone feed back to inhibit the anterior pituitary and hypothalamus
• Conversion of T4 to T3

  • Most T4 is converted to T3 (the more active form) in the target tissues
  • These hormones act by turning on gene transcription and protein synthesis