Endocrine System and Hormones

Hormone Mechanisms

  • Hormones travel through the bloodstream and target specific tissues or cells.
  • Hormone shape matches a protein receptor on the target cell surface, similar to an enzyme and substrate.
  • This match causes a change in the receptor protein, triggering a response inside the cell.
  • Hormones control:
    • Reproductive responses
    • Growth
    • Metabolism (sugar usage rate)
    • Blood pressure
    • Brain responses

Hormone Release

  • Hormone secretion is controlled by:
    • Nerve impulses
    • Changes in ion concentration outside the cell
    • Other hormones (cascade effect)

Types of Hormones

  • Two types of hormones:
    • Protein-based/peptide hormones
    • Lipid-based hormones (often have a steroid structure - four fused carbon rings)

Hormone Action

  • Peptide hormones:
    • Interact with receptors on the cell surface.
    • The receptor shape change triggers a cascade inside the cell cytoplasm, causing a cellular response.
    • The peptide hormone stays outside the cell.
  • Lipid hormones:
    • Diffuse through the phospholipid membrane into the cell.
    • Attach to a receptor protein inside the cytoplasm.
    • The hormone-receptor complex enters the nucleus and influences gene transcription.
  • Analogy:
    • Peptide hormone: Knocks on the door and asks for a peanut butter sandwich.
    • Lipid hormone: Busts down the door and makes the sandwich itself. (more direct response)

Hormone Examples

  • Epinephrine (adrenaline): A water-soluble, peptide hormone, found in EpiPens for allergies.
  • Sex hormones (testosterone, estrogen): Lipid-based hormones.

Endocrine Glands

  • Key endocrine glands:
    • Hypothalamus
    • Pituitary gland
    • Thyroid gland
    • Adrenal glands
    • Pancreas
    • Ovaries and Testes

Hypothalamus

  • Located deep inside the brain.
  • A link between the nervous system and the endocrine system.
  • Coordinates nervous system responses.
  • Controls the pituitary gland.
  • Secretes neurohormones.
  • Often the starting point for hormone cascades.

Thyroid Hormone Cascade Example

  • Hypothalamus releases TRH (thyroid releasing hormone).
  • TRH receptors are in the pituitary gland.
  • Pituitary gland releases TSH (thyroid stimulating hormone).
  • TSH goes to the thyroid gland.
  • Thyroid gland releases T3 and T4 (thyroid hormones).
  • Feedback response: The body detects a rise in thyroid hormones, and the hypothalamus releases less TRH.

Pituitary Gland

  • Two glands in one: anterior and posterior lobes.
  • Anterior pituitary releases:
    • Growth hormone
    • Hormones that affect testes and ovaries
    • Prolactin (targets mammary glands)
    • Melanocyte stimulating hormone
  • Posterior pituitary releases:
    • Oxytocin (targets uterus muscles)
    • ADH (antidiuretic hormone, targets kidney tubules for water balance)

Posterior Pituitary

  • Stores oxytocin and ADH, which are synthesized in the hypothalamus.
  • Hormones are transported down special neurons and stored in the pituitary gland.
  • Release is triggered by a nerve impulse from the hypothalamus.

Anterior Pituitary

  • Synthesizes its own hormones (growth hormone, sex hormones, thyroid hormones).
  • Controlled by the hypothalamus via releasing hormones.
  • Example: Growth hormone releasing hormone (GHRH) from the hypothalamus triggers the anterior lobe to release growth hormone.

Comparison of Pituitary Lobes

  • Posterior: Hormones created in hypothalamus, released by nerve impulse.
  • Anterior: Hormones created in anterior lobe, triggered for release by a hormone from the hypothalamus.

Thyroid Gland

  • Butterfly-shaped organ wrapping around the trachea.
  • Synthesizes T3 (triiodothyronine) and T4 (thyroxine).
  • Requires iodine for synthesis.
  • Primarily regulates metabolic rates and nutrient metabolism.
  • Also affects blood flow and temperature perception.

Thyroid Disorders

  • Hypothyroidism: Too low thyroid hormone production.
  • Hyperthyroidism: Too high thyroid hormone production.

Hypothyroidism Symptoms

  • Lower than normal metabolic rate
  • Decreased appetite
  • Weight gain
  • Cold intolerance

Hyperthyroidism Symptoms

  • Higher than normal metabolic rate
  • Increased appetite
  • Weight loss
  • Increased temperature
  • Heat intolerance

Thyroid Feedback Loop

  • Hypothalamus secretes TRH.
  • Anterior pituitary secretes TSH.
  • Thyroid gland secretes T3 and T4.
  • T3 and T4 levels feed back to the pituitary and hypothalamus, regulating TRH and TSH secretion.

Iodine Deficiency and Goiter

  • If there is not enough iodine: then Thyroid gland cannot create thyroid hormones.
  • Concentration of thyroid hormones will drop.
  • Hypothalamus release more TRH to stimulate TSH.
  • Thyroid gland gets larger (goiter) in attempt to produce more hormones.