Endocrine Control of Growth and Metabolism: Study Notes

Review of Endocrine Principles

The Hypothalamic-Pituitary Control System

The hypothalamic-pituitary axis is a central regulatory system for many endocrine glands. It involves the release of hormones from the hypothalamus that control the secretion of pituitary hormones, which in turn regulate peripheral endocrine glands.

• Feedback Patterns: Most endocrine systems operate via negative feedback loops, where the final hormone inhibits its own production.

• Hormone Receptors: Target cells must express specific receptors to respond to hormones.

• Cellular Responses: Hormones can alter gene expression, enzyme activity, or membrane transport in target cells.

• Modulation of Target Cell Response: Sensitivity and responsiveness can be adjusted by receptor number or affinity.

• Endocrine Pathologies: Disorders may result from hormone excess, deficiency, or abnormal tissue responsiveness.

Adrenal Glucocorticoids

Structure and Function of the Adrenal Glands

The adrenal glands are paired organs located atop the kidneys, consisting of two distinct regions: the adrenal cortex and adrenal medulla.

• Adrenal Medulla: Secretes catecholamines (e.g., epinephrine) for rapid fight-or-flight responses.

• Adrenal Cortex: Secretes steroid hormones and is divided into three zones:

  • Zona Glomerulosa: Secretes aldosterone (mineralocorticoid).

  • Zona Fasciculata: Secretes glucocorticoids (mainly cortisol).

  • Zona Reticularis: Secretes sex hormones (androgens).

• Hormone Crossover Effects: Structural similarities among steroids can lead to overlapping receptor binding and symptoms.

Cortisol Secretion and Regulation

Cortisol is the primary glucocorticoid, essential for life and stress adaptation. Its secretion is regulated by the hypothalamic-pituitary-adrenal (HPA) axis.

• Control Pathway: CRH (hypothalamus) → ACTH (anterior pituitary) → Cortisol (adrenal cortex).

• Negative Feedback: Cortisol inhibits CRH and ACTH secretion.

• Diurnal Rhythm: Cortisol peaks in the morning and declines at night; increases with stress.

Physiological Effects of Cortisol

• Metabolic Effects: Promotes gluconeogenesis, protein catabolism, and lipolysis; increases blood glucose.

• Immune Suppression: Inhibits cytokine release, antibody production, and inflammation.

• Calcium Balance: Decreases intestinal absorption and increases renal excretion of Ca2+, leading to net loss.

• Brain Function: Affects mood, memory, and learning.

Therapeutic Uses and Pathologies of Cortisol

• Therapeutic Use: Used as an immunosuppressant and anti-inflammatory agent.

• Adverse Effects: Long-term use can suppress ACTH, causing adrenal atrophy; must taper off gradually.

• Hypercortisolism (Cushing’s Syndrome): Causes hyperglycemia, muscle wasting, central obesity, and mood changes. May result from tumors or exogenous steroids.

• Adrenal Insufficiency (Addison’s Disease): Hyposecretion of adrenal steroids, often autoimmune; can cause hyperpigmentation due to increased ACTH.

• Congenital Adrenal Hyperplasia: Enzyme defects lead to excess androgen production and possible masculinization in females.

Thyroid Hormones

Structure and Synthesis

The thyroid gland, located at the base of the throat, produces hormones essential for metabolism and development. It contains follicular cells (secrete thyroid hormone) and C cells (secrete calcitonin).

• Thyroid Hormones: Derived from tyrosine and iodine (T3 and T4).

• Synthesis Steps:

  • Thyroglobulin and enzymes secreted into follicle colloid.

  • Iodide actively transported into follicle, oxidized, and added to tyrosine (forming MIT and DIT).

  • Coupling reactions produce T3 (MIT + DIT) and T4 (DIT + DIT).

  • Hormones released into plasma, mostly as T4, bound to thyroid-binding globulin (TBG).

  • T3 is the active form; most T3 is produced from T4 in target tissues by deiodinases.

Regulation and Actions of Thyroid Hormones

• Control Pathway: TRH (hypothalamus) → TSH (anterior pituitary) → Thyroid hormones (thyroid gland).

• Functions: Increase basal metabolic rate, oxygen consumption, and are thermogenic. Essential for growth and nervous system development in children.

Thyroid Pathologies

• Goiter: Enlargement of the thyroid due to elevated TSH, not specific to cause.

• Hyperthyroidism: Excess hormone causes increased metabolism, heat intolerance, muscle wasting, and psychological symptoms. Graves’ disease is an autoimmune cause.

• Hypothyroidism: Deficiency slows metabolism, causes cold intolerance, brittle hair/nails, and developmental delays in children (cretinism). Often due to iodine deficiency.

Growth Hormone (GH)

Regulation and Actions

Growth hormone is essential for normal growth and metabolism. Its secretion is regulated by GHRH and somatostatin from the hypothalamus.

• GH in Blood: About half is bound to plasma proteins, extending its half-life.

• IGFs: GH stimulates the liver to produce insulin-like growth factors, which mediate many growth-promoting effects.

• Metabolic Effects: Promotes protein synthesis, lipolysis, and gluconeogenesis; increases blood glucose.

• Bone and Cartilage: GH and IGFs stimulate bone growth; IGFs directly stimulate cartilage synthesis.

Growth Disorders

• Dwarfism: GH deficiency or receptor defects in children.

• Giantism: GH excess before epiphyseal closure in children.

• Acromegaly: GH excess in adults, causing enlarged jaw, hands, and feet.

Tissues and Bone Growth

Types of Growth

• Soft Tissue Growth: Involves hypertrophy (cell size increase) and hyperplasia (cell number increase); requires GH, thyroid hormone, and insulin.

• Linear Bone Growth: Occurs at epiphyseal plates in children; requires protein and calcium. Bone growth ceases after adolescence but remodeling continues throughout life.

Calcium Balance

Physiological Roles of Calcium

• Signaling: Ca2+ is a key intracellular messenger.

• Structural: Part of intercellular cement and bone matrix.

• Coagulation: Essential for blood clotting.

• Neuronal Excitability: Plasma Ca2+ levels affect nerve and muscle function.

Calcium Homeostasis

• Distribution: 99% in bone, rest in extracellular fluid and cells.

• Intake: From diet, absorbed in small intestine (hormonally regulated).

• Output: Mainly via kidneys, some in feces.

Hormonal Regulation of Calcium

• Parathyroid Hormone (PTH): Increases plasma Ca2+ by mobilizing bone calcium, enhancing renal reabsorption, and increasing intestinal absorption (via calcitriol).

• Calcitriol (Vitamin D3): Enhances intestinal absorption of calcium; synthesized from dietary or skin-derived vitamin D, activated in liver and kidneys.

• Calcitonin: Lowers plasma Ca2+; released when calcium is high.

Bone Remodeling and Disorders

Bone Cells and Remodeling

• Osteoblasts: Build bone matrix.

• Osteoclasts: Resorb bone, releasing calcium and phosphate.

• Phosphate: Important for bone mineralization and energy transfer.

Osteoporosis

• Definition: Metabolic disorder where bone resorption exceeds deposition, leading to fragile bones.