Endocrine System: ADH, Thyroid, and Adrenal Glands Study Guide

Function and Location:
- Stimulates the reabsorption of water in the distal and collecting tubules of the kidney.
- Also affects the salivary glands and the small intestine.
- Its primary action results in a decrease of diuresis (urine production).
Mechanism of Action:
- Opens specific water channels known as aquaporins.
- The movement of water through these channels is passive, driven by the osmotic gradient.
Physiological Impact:
- Regulates approximately $10-15\%$ of the water reabsorbed in the kidneys.
- This accounts for a volume of roughly $17-18\,dm^3/24\,h$ .
Vascular Effect:
- Acts as a vasopressor, constricting the blood vessels (hence the name vasopressin).
Regulation of Secretion:
1. Osmolality: Release is stimulated by an increase in blood osmolality (e.g., after consuming salty fish). Osmotic receptors located in the hypothalamus are positioned near the ADH-producing cells.
2. Volume Receptors: Secretion is triggered following a $10-15\%$ decrease in blood volume.
3. Alcohol: Known to decrease ADH secretion, leading to increased diuresis.
4. Nicotine: Acts as a stimulant for ADH secretion.
Pathology:
- ADH Insufficiency (Diabetes Insipidus): Characterized by excessive diuresis, with urine output reaching $18-20\,dm^3/24\,h$ .

Microscopic Structure:
- Follicles: Lined with cuboidal epithelium.
- Colloid: Stored within the follicles; contains thyroglobulin, the precursor for thyroid hormones.
- Parafollicular Cells: Also known as C-cells, located between the follicles.
Hormone Types:
- The gland releases two iodine-containing follicular hormones synthesized from iodine and the amino acid tyrosine:
  - Thyroxine (Tetraiodothyronine) - $T_4$
  - Triiodothyronine - $T_3$
- It also releases one non-iodine-containing hormone from the parafollicular cells:
  - Calcitonin: Essential for calcium ( $Ca$ ) and phosphorus ( $P$ ) homeostasis.

Metabolic Effects:
- Increases tissue oxygen ( $O_2$ ) consumption.
- Calorigenic Effect: Increases heat production and the Basal Metabolic Rate (BMR).
- Protein Metabolism:
  - Within normal physiological ranges, these hormones have an anabolic effect.
  - Levels higher than normal result in a catabolic effect.
Growth and Development:
- Essential for the development and normal functioning of the Central Nervous System (CNS); deficiency can lead to mood changes or cretinism.
- Critical for normal physical growth; it is considered the most important hormone for body and mental development during the first year of a child's life.
Systemic Effects:
- Carbohydrate Metabolism: Exhibits a hyperglycemic effect.
- Lipid Metabolism: Exhibits a lipolytic effect.
- Cardiovascular: Increases the sensitivity of the heart to catecholamines, which can lead to a condition known as cor thyreotoxicum.
- Muscular: Affects the velocity of muscle contraction.
- Digestive: Regulates the motor functions of the Gastrointestinal Tract (GIT).

Regulation of Secretion:
- Hypothalamus: Releases TRH (Thyrotropin-Releasing Hormone) stimulated by birth, stress, or temperature changes.
- Pituitary Gland: TRH stimulates the pituitary to release TSH (Thyroid-Stimulating Hormone).
- Thyroid Gland: TSH stimulates the production of $T_4$ and $T_3$ .
- Negative Feedback Loop: High levels of thyroid hormones suppress the release of TRH and TSH; low levels stimulate them.
- Peripheral Tissues: Much of $T_4$ is converted to the more active $T_3$ in the periphery.
Clinical Disorders:
- Hypothyroidism (Decreased Production):
  - In children: Leads to nanism (dwarfism) accompanied by cretinism.
  - In adults: Results in myxedema.
- Hyperthyroidism (Increased Production):
  - Conditions include Thyrotoxicosis and Grave disease.
  - Goiter: An enlargement of the thyroid gland, which can be endemic.

Adrenal Cortex Zones:
- Zona Glomerulosa: Produces mineralocorticoids.
- Zona Fasciculata: Produces glucocorticoids and sex hormones.
- Zona Reticularis: Produces glucocorticoids and sex hormones.
Adrenal Medulla:
- Described as a modified sympathetic ganglion.
- Functions as part of the sympathetic-adrenal system, innervated by preganglionic sympathetic neurons.
- Termed "liquid sympathicus" due to its hormonal release into the blood.
- Secretions:
  - Epinephrine (Adrenalin): $80\%$
  - Norepinephrine (Noradrenalin): $20\%$
  - Also secretes Enkephalins and Endorphins.
- Role: Mediates the stress response via adrenoceptors.

Primary Hormones: Cortisol and Corticosterone.
Metabolism: Degraded in the liver into cortisone.
Metabolic and Vascular Effects:
- Hyperglycemic Effect: Increases gluconeogenesis from amino acids.
- Catabolic and Lipolytic Effects: Breaks down proteins and fats.
- Vascular Sensitivity: Increases the sensitivity of blood vessels to catecholamines.
Digestive and Blood Effects:
- Digestive: Stimulates secretion of digestive juices but decreases mucosal defense mechanisms, leading to an ulcer effect.
- Blood Changes:
  - Decreased Eosinophils ( $Eo$ ) and lymphocytes.
  - Decrease in lymph node size/activity.
  - Increase in Red Blood Cell (RBC) count.
  - Increase in hemocoagulation (blood clotting).
Immune and Bone Effects:
- Suppresses immune reactions (primary reason for use in treating allergies).
- Suppresses bone formation; prolonged therapy can lead to osteoporosis or decreased height in children.

Therapeutic Uses:
- Anti-inflammatory and anti-proliferative effects.
- Immunosuppressive effects for allergic and autoimmune conditions.
Clinical Cautions (The "Why" of Dose-schemas):
- Risk of Osteoporosis.
- Risk of Steroid Diabetes.
- Risk of Ulcers.
- Risk of Hypertension.
Regulation of Secretion:
- Hypothalamus: Releases CRF (Corticotropin-Releasing Factor), stimulated by stress and circadian rhythms.
- Pituitary: CRF stimulates the release of ACTH (Adrenocorticotropic Hormone).
- Adrenal Cortex: ACTH stimulates the cortex to release Cortisol.
- Negative Feedback: Cortisol inhibits both the hypothalamus and pituitary.
Diurnal Rhythm:
- Cortisol levels oscillate throughout the day, with a significant secretory surge approximately one hour after awakening in the morning.
Pathology:
- Cushing Disease (Hyperadrenocorticism):
  - Symptoms include facies lunata ("moon face") and fat pads on the back of the neck ("buffalo hump").
  - Associated with hypertension, steroid diabetes, and osteoporosis.