Endocrine System: Thyroid, Parathyroid, Adrenal, and Pineal Glands Study Notes

Anatomy and Physiology of the Thyroid Gland

• Location and Structure

  • The thyroid gland is situated inferior to the thyroid cartilage of the larynx.

  • It consists of two lobes connected by a narrow isthmus, which is a joining of tissue.

  • It is highly vascular and sits well-protected underneath the cricoid cartilage and the larynx (voice box).

  • The gland contains follicles, which are hollow spheres lined with cuboidal epithelium.

  • The follicle cavities contain a viscous substance called colloid.

  • It also contains C cells, which are also known as clear cells or parafollicular cells.

• Thyroid Hormones and Iodine

  • The gland is involved in the secretion of thyroglobulin.

  • Thyroglobulin contains the amino acid tyrosine, which serves as the building block for thyroid hormones.

  • T4 (Thyroxine): Contains four iodine atoms.

  • T3 (Triiodothyronine): Contains three iodine atoms.

  • Nutritional Context: In Australia, people typically get sufficient iodine intake from their diet. In regions where iodine is low, the thyroid gland can undergo overgrowth, a condition known as a goiter.

• Homeostasis of Thyroid Secretion

  • Disturbance: Homeostasis is disturbed when blood concentrations of $T3$ and $T4$ decrease or when body temperature is low.

  • Hypothalamic Response: The hypothalamus is stimulated to release Thyroid Releasing Hormone ($TRH$).

  • Pituitary Response: $TRH$ stimulates the anterior lobe of the pituitary gland to release Thyroid Stimulating Hormone ($TSH$).

  • Thyroid Action: $TSH$ acts on the thyroid gland, triggering the release of $T3$ and $T4$.

  • Restoration: This elevates the concentration of thyroid hormones in the blood and restores homeostasis via a negative feedback loop.

• Cellular and Developmental Effects

  • Thyroid hormones affect almost every cell in the body.

  • They bind to receptors in three specific locations: the cytoplasm, on the surface of the mitochondria, and in the nucleus.

  • In children, these hormones are essential for the normal development of the skeletal muscle and nervous systems.

• Additional Physiological Effects

  • Elevates oxygen and energy consumption; in children, this may cause a rise in body temperature.

  • Increases heart rate and the force of contraction.

  • Increases sensitivity to sympathetic stimulation (the "fight or flight" response).

  • Maintains normal sensitivity of respiratory centers to oxygen and carbon dioxide concentrations.

  • Stimulates red blood cell ($RBC$) formation.

  • Stimulates activity in other endocrine tissues.

  • Accelerates the turnover of minerals in bones.

The Parathyroid Glands and Calcium Regulation

• Anatomy and Mass

  • These consist of two pairs of glands (four glands in total) embedded on the posterior surface of the thyroid gland.

  • Collectively, the four glands weigh only $1.6\,g$.

• Parathyroid Hormone (PTH)

  • Secreted by parathyroid principal cells in response to low concentrations of calcium ions in the blood.

  • It acts as the antagonist to calcitonin, which is produced by the thyroid gland.

• Mechanism of Action

  • Osteoclast Stimulation: Stimulates osteoclasts (one of the three bone cell types alongside blasts and sites) to accelerate the destruction of bone matrix, releasing calcium into the blood.

  • Renal Effects: Enhances the reabsorption of calcium by the kidneys, thereby reducing loss in urine.

  • Calcitriol Formation: Stimulates the formation and secretion of calcitriol by the kidneys.

• Calcium Homeostasis Targets

  • Intestines: Site for calcium absorption from diet into the blood.

  • Kidneys: Excrete calcium from blood via urine and produce calcitriol.

  • Bone: Serves as a reservoir for storing calcium.

Homeostatic Mechanisms for Blood Calcium

• Scenario A: High Blood Calcium Levels

  • Threshold: Levels rise above $11\,mg/dl$.

  • Stimulus: C cells in the thyroid gland are stimulated.

  • Hormone Released: Calcitonin ($CT$).

  • Target Organs and Responses:

    • Kidneys: Increase excretion of calcium; reduce renal production of calcitriol.

    • Digestive System: Decrease calcium reabsorption in the digestive tract.

    • Bones: Calcitonin inhibits osteoclasts (without affecting osteoblasts), leading to increased calcium deposition from blood into bone.

  • Normal Range: Homeostasis aims for $8.5$ to $11\,mg/dl$.

• Scenario B: Low Blood Calcium Levels

  • Threshold: Levels fall below $8.5\,mg/dl$.

  • Stimulus: Receptor cells of the parathyroid glands are stimulated.

  • Hormone Released: Parathyroid Hormone ($PTH$).

  • Target Organs and Responses:

    • Kidneys: Increase reabsorption of calcium and increase calcitriol production.

    • Bones: Increased osteoclast activity releases stored calcium into the blood.

    • Digestive System: Increased calcitriol enhances calcium absorptionจาก the food we eat.

• Clinical Significance

  • Calcium is vital for muscle contractions, nerve impulses, and blood clotting.

  • Medical application: Pharmacological calcitonin is used as a treatment for osteoporosis in postmenopausal women because it inhibits osteoclastic activity.

The Adrenal (Suprarenal) Glands

• Location and Division

  • Situated on the superior border of the kidneys.

  • Separated into the superficial adrenal cortex and the inner adrenal medulla.

• The Adrenal Cortex

  • Stores lipids, specifically cholesterol and fatty acids.

  • Manufactures steroid hormones known as corticosteroids, which are fat-soluble.

  • Zone 1: Zona Glomerulosa (Outer): Produces mineralocorticoids, primarily aldosterone.

    • Stimulates sodium ion conservation and potassium ion elimination.

    • Increases sensitivity of salt receptors in taste buds.

    • Secreted in response to a drop in blood sodium, volume, or pressure.

  • Zone 2: Zona Fasciculata (Middle): Produces glucocorticoids ($cortisol$, $corticosteroid$, and $cortisone$).

    • Known as stress hormones.

    • Regulated by negative feedback; they inhibit the production of Corticotrophin Releasing Hormone ($CRH$) in the hypothalamus and Adrenocorticotropic Hormone ($ACTH$) in the anterior pituitary.

    • Accelerates glucose synthesis and glycogen formation in the liver.

    • Anti-inflammatory effects: Inhibits white blood cell activity and immune system components.

    • Clinical Note: Synthetic glucocorticoids (e.g., Prednisolone) are prescribed to decrease inflammation, such as in asthma exacerbations.

  • Zone 3: Zona Reticularis (Inner): Forms a narrow band and produces small quantities of androgens.

    • Stimulated by $ACTH$.

    • Some are converted to estrogens in the blood.

    • Stimulate development of pubic hair before puberty.

• The Adrenal Medulla

  • Controlled by the sympathetic division of the autonomic nervous system.

  • Secretory Cells and Hormones:

    • $75-80\%$ Epinephrine (Adrenaline).

    • $20-25\%$ Norepinephrine (Noradrenaline).

  • These are collectively called catecholamines.

  • Activation Effects (Fight or Flight Response):

    • Skeletal Muscles: Trigger mobilization of glycogen reserves and accelerate glucose breakdown to help work more efficiently (e.g., if being charged by a lion).

    • Adipose Tissue: Break down stored fats into fatty acids for energy.

    • Liver: Break down glycogen molecules.

    • Heart: Stimulate beta-1 receptors to increase the strength and rate of cardiac contraction.

The Pineal Gland

• Location and Function

  • Located in the posterior portion of the roof of the third ventricle of the brain.

  • Contains secretory cells called pinealocytes.

  • Hormone Produced: Melatonin.

• Physiological Roles of Melatonin

  • Influences circadian rhythms (sleep-wake cycles).

  • Inhibits reproductive function.

  • Protects the body against damage from free radicals.

Questions & Discussion

• Q: Why is understanding calcium homeostasis important?

  • A: Calcium is essential for practical physiological processes including muscle contraction, nerve impulse transmission, and blood clotting. Furthermore, understanding the role of calcitonin has led to pharmacological treatments for osteoporosis.

• Q: What is the significance of the sympathetic nervous system in the adrenal gland?

  • A: The sympathetic division specifically controls the adrenal medulla. During a fight-or-flight response, it triggers the release of adrenaline and noradrenaline, which maximize survival by increasing energy availability and heart performance.