Endocrine

Overview of Homeostasis

  • The body maintains homeostasis through two primary systems: the nervous system and the endocrine system.
    • These systems work together to optimize physiological functions.
  • The endocrine system can be compared to a wireless system, where information is relayed through hormones that circulate in the blood.
  • Feedback loops play a crucial role in regulating hormonal levels:
    • Negative feedback: The response counteracts the initiating stimulus to restore a parameter to a set point, maintaining stability (homeostasis).
    • Positive feedback: The response amplifies the original stimulus, creating an unstable cycle that increases the magnitude of the response rather than stabilizing it.

Hormonal Functions

  • Autocrine function: Hormone acts at the site of origin.
  • Paracrine function: Hormone acts on adjacent cells.
  • Endocrine function: Hormone enters the bloodstream and acts at distant sites.

Endocrine vs. Nervous System Communication

  • Nervous System:

    • Mode: Wired
    • Messenger: Electrical (via neurotransmitters)
    • Distance: Local (synaptic)
    • Speed: Fast
    • Duration: Short

  • Endocrine System:

    • Mode: Wireless
    • Messenger: Hormones
    • Distance: Systemic (travels in blood)
    • Speed: Slow
    • Duration: Long

  • Various receptors for endocrine functions include ion channels, G-proteins, enzymes, and gene activation.

Feedback Loops in the Endocrine System

  • Negative Feedback Mechanism:

    • Example: Hypothalamus detects high hormone levels and signals the pituitary to decrease hormone secretion.
    • Short Loop: Involves the hypothalamus and anterior pituitary.
    • Long Loop: Involves the endocrine gland and target tissue.

  • Positive Feedback Mechanism:

    • Example: Increased oxytocin during childbirth leads to uterine contractions, which trigger more oxytocin release.

Hypothalamic-Pituitary Axis

  1. The hypothalamus connects the central nervous system to the endocrine system by monitoring hormone levels in the blood.
    • It regulates the pituitary gland's hormone release through releasing and inhibiting hormones.
    • The hypothalamus is located outside of the blood-brain barrier, allowing direct interaction with the bloodstream.
  2. The pituitary gland is divided into anterior and posterior segments.

Anterior Pituitary Hormones

  • Follicle-Stimulating Hormone (FSH): Germ cell maturation and ovarian follicle growth in females.
  • Luteinizing Hormone (LH): Testosterone production in males and ovulation in females.
  • Adrenocorticotropic Hormone (ACTH): Stimulates adrenal hormone release.
  • Thyroid-Stimulating Hormone (TSH): Stimulates thyroid hormone release.
  • Prolactin: Promotes lactation.
  • Growth Hormone (GH): Stimulates cell growth.

Posterior Pituitary Hormones

  • Antidiuretic Hormone (ADH): Promotes water retention.
  • Oxytocin: Facilitates uterine contractions and breastfeeding.

Hormonal Regulation and Disorders

  1. Negative Feedback Regulation: Hormone levels directly affect the hypothalamic-pituitary axis.

Conditions Related to Hormonal Dysregulation

  • SIADH (Syndrome of Inappropriate ADH Secretion): Excessive levels of ADH lead to low urine output and possible hyponatremia.

    • Commonly caused by traumatic brain injuries.
    • Treatment includes fluid restriction and medications like demeclocycline or hypertonic saline.

  • Diabetes Insipidus (DI): Characterized by insufficient ADH leading to high urine output.

    • Commonly linked to pituitary surgery.
    • Treated with DDAVP or supportive measures.

  • Acromegaly: Caused by GH overproduction post-adolescence, often from a pituitary adenoma. Key considerations include:

    • Distorted facial features may complicate airway management.
    • Comorbidities include OSA, CAD, and entrapment neuropathies.

Thyroid Anatomy & Physiology

  1. Thyroids secrete:
    • T4 (Thyroxine): Prohormone synthesized from tyrosine.
    • T3 (Triiodothyronine): The active thyroid hormone.
    • Calcitonin: Lowers serum calcium levels.
  2. The thyroid requires iodine for T3 and T4 synthesis.
  3. Thyroid Hormone Physiology:
    • Overall effects include increased basal metabolic rate (BMR) leading to increased oxygen and CO2 production.

Thyroid Disorders

  • Hyperthyroidism: Commonly caused by Grave's disease or thyroid storm. Symptoms include weight loss, increased appetite, and anxiety.
  • Hypothyroidism: Most often caused by Hashimoto's thyroiditis or iodine deficiency. Symptoms include weight gain, fatigue, and cold intolerance.

Diabetes Mellitus

  1. Defined as an inability to adequately utilize glucose due to lack of insulin (Type 1) or insulin resistance (Type 2).

  2. Diagnostic Criteria for diabetes include elevated fasting plasma glucose levels and symptomatology such as polyuria, polydipsia, and dehydration.

  3. Complications include:

    • Diabetic ketoacidosis (DKA): More frequent in Type 1, treated through fluid resuscitation and insulin therapy.
    • Hyperglycemic hyperosmolar state (HHS): More typical in Type 2, resulting from insulin deficiency.

Anesthetic Considerations

  1. Diabetes mellitus poses risks including orthostatic hypotension, aspiration, and hypothermia.
  2. Medical management prior to surgery involves controlling blood glucose levels and possibly using intravenous insulin.
  3. Special consideration should be given to the potential for hypoglycemia during surgery, especially in patients on beta-blockers or those who are autonomically neuropathic.

Pancreas Physiology

  1. The pancreas has both exocrine (digestion) and endocrine (metabolism) functions, with hormones such as insulin (lowers blood glucose) and glucagon (raises blood glucose).
  2. Insulin Release Mechanism: Glucose stimulates beta cells to release insulin, promoting glucose uptake by tissues.

Oral Hypoglycemic Medications

  1. Categories include Biguanides, Sulfonylureas, Meglitinides, and Thiazolidinediones.
  2. Key considerations during perioperative management include recognizing the risk of lactic acidosis with Metformin and the interaction with other medications leading to hypoglycemia.

Adrenal Physiology

  1. The adrenal glands produce hormones such as aldosterone (mineralocorticoid) and cortisol (glucocorticoid).
  2. Cushing's syndrome results from excessive cortisol, while Addison's disease results from insufficient hormone production.
  3. Anesthetic considerations are essential due to potential steroid requirements and careful management to prevent adrenal crisis during stressful conditions.