Endocrine Control of Growth and Metabolism: Chapter 23 Study Notes

Endocrine Control of Growth and Metabolism

23.1 Review of Endocrine Principles

The endocrine system regulates physiological processes through hormones, which are chemical messengers secreted into the blood. This section reviews the foundational principles of endocrine control.

• Hypothalamic-pituitary control system: The hypothalamus and pituitary gland coordinate the release of many hormones, often through hierarchical pathways.

• Feedback patterns: Most endocrine pathways use negative feedback to maintain homeostasis, where hormone levels regulate their 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 response can be modulated by receptor number and affinity.

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

23.2 Adrenal Glucocorticoids

Adrenal Cortex Hormones

The adrenal cortex produces several steroid hormones, each with distinct physiological roles.

• Aldosterone: Regulates sodium-potassium balance; secreted by the zona glomerulosa (outer layer).

• Androgens: Produced in the zona reticularis (inner layer); contribute to secondary sex characteristics.

• Glucocorticoids: Mainly cortisol, secreted by the zona fasciculata (middle layer); involved in stress response and metabolism.

Adrenal Gland Structure and Function

Region	Secretes
Zona glomerulosa	Aldosterone
Zona fasciculata	Glucocorticoids (Cortisol)
Zona reticularis	Androgens
Adrenal medulla	Catecholamines (e.g., epinephrine)

Cortisol Secretion and Regulation

Cortisol is regulated by the hypothalamic-pituitary-adrenal (HPA) axis:

• Pathway: Corticotropin-releasing hormone (CRH) → adrenocorticotropic hormone (ACTH) → cortisol

• Continuous secretion: Follows a diurnal rhythm (higher in the morning).

• Transport: Carried in blood by corticosteroid-binding globulin (CBG).

Functions of Cortisol

• Stress mediation: Essential for long-term stress adaptation.

• Permissive effect: Protects against hypoglycemia.

• Metabolic effects (catabolic):

  • Promotes gluconeogenesis (glucose synthesis from non-carbohydrate sources).

  • Breaks down skeletal muscle proteins.

  • Enhances lipolysis (fat breakdown).

  • Suppresses the immune system.

  • Causes negative calcium balance (Ca2+ loss).

  • Influences brain function.

Cortisol as a Therapeutic Drug

• Immunosuppressant: Inhibits immune and inflammatory responses; used to treat allergies and prevent organ transplant rejection.

• Risks: Exogenous cortisol can suppress ACTH production, leading to adrenal cell atrophy.

Cortisol Pathologies

• Hypercortisolism:

  • Primary: Adrenal tumor secreting cortisol.

  • Secondary: Pituitary tumor secreting ACTH (Cushing's disease).

  • Iatrogenic: Procedure-induced (Cushing's syndrome).

• Hypocortisolism: Addison's disease – hyposecretion of all adrenal steroid hormones.

23.3 Thyroid Hormones

Thyroid Hormone Synthesis and Function

Thyroid hormones have long-term effects on metabolism and are essential for growth and development in children and infants.

• Synthesis: Occurs in thyroid follicles; involves iodination of tyrosine residues on thyroglobulin.

• Types: Monoiodotyrosine (MIT), diiodotyrosine (DIT), triiodothyronine (T3), thyroxine (T4).

• Transport: Bind to plasma proteins such as thyroid-binding globulin (TBG).

• Activation: Target cells use deiodinases to convert T4 to active T3.

Regulation by TSH

• Pathway: Thyrotropin-releasing hormone (TRH) → thyroid-stimulating hormone (TSH) → thyroid hormones (T3 & T4).

• Functions: Promote oxidative metabolism, influence metabolism, and are necessary for full expression of growth hormone in children.

Thyroid Hormone Table

Cell of Origin	Thyroid follicle cells
Chemical Nature	Iodinated amine
Biosynthesis	From iodine and tyrosine; stored on thyroglobulin in follicle colloid
Transport	Bound to thyroid-binding globulin and albumin
Target Cells	Most cells of the body
Action at Cellular Level	Increases activity of metabolic enzymes and Na+-K+ ATPase

Thyroid Pathologies

• Goiter: Enlarged thyroid gland due to excessive stimulation.

• Hyperthyroidism:

  • Increased oxygen consumption and metabolic heat production.

  • Increased protein catabolism, muscle weakness.

  • Hyperexcitable reflexes, psychological disturbances.

  • Exophthalmos (bulging eyes) in Graves' disease.

• Hypothyroidism:

  • Slowed metabolic rate and oxygen consumption.

  • Decreased protein synthesis, myxedema (puffy appearance).

  • Slowed reflexes, speech, thought processes, fatigue.

  • Cretinism in infancy (congenital hypothyroidism).

  • Bradycardia (slow heart rate).

• Treatment: Oral thyroid hormone for hypothyroidism; gland removal, radioactive iodine, or synthesis blockers for hyperthyroidism.

23.4 Growth Hormone

Growth Hormone Function and Regulation

Growth hormone (GH) is essential for normal growth and development, acting through direct effects and by stimulating insulin-like growth factors (IGFs).

• Normal growth depends on:

  • GH and other hormones

  • Adequate diet

  • Absence of chronic stress

  • Genetics

• GH is anabolic:

  • Stimulates IGF secretion

  • Promotes protein synthesis

  • Increases bone growth

  • Increases lipolysis and gluconeogenesis, raising blood glucose

Feedback Control of GH Secretion

• GH and IGFs inhibit GHRH (negative feedback) and promote somatostatin (GHIH) release.

• IGFs directly inhibit GH secretion.

• Pathologies:

  • Severe GH deficiency in childhood: dwarfism

  • Oversecretion in children: gigantism

  • Excessive secretion in adults: acromegaly

23.5 Tissue and Bone Growth

Bone Growth and Remodeling

Bone growth is regulated by hormones and requires adequate dietary calcium. Bone tissue is remodeled throughout life.

• Hydroxyapatite: Main form of calcium phosphate in bone.

• Growth process: Matrix deposition exceeds resorption during growth.

• Osteoblasts: Produce enzymes and osteoid (organic matrix).

• Osteoclasts: Secrete acid to dissolve calcified matrix.

• Growth at epiphyseal plates: Chondrocytes form cartilage, replaced by bone.

• Osteoblasts revert to osteocytes: Less active, maintain bone tissue.

23.6 Calcium Balance

Importance of Calcium

Calcium ions (Ca2+) are vital for signaling, cell adhesion, blood clotting, and muscle/nerve function.

• Signal molecule: Involved in neurotransmitter release and muscle contraction.

• Cell adhesion: Part of the 'cement' at tight junctions.

• Coagulation: Cofactor in the clotting cascade.

• Excitability: Plasma Ca2+ affects neuron and muscle excitability; imbalance leads to hypocalcemia or hypercalcemia.

Calcium Distribution Table

Location	Function
Extracellular Matrix (99%)	Calcified matrix of bone and teeth
Extracellular Fluid (0.1%)	Neurotransmitter release, muscle contraction, coagulation, cell adhesion
Intracellular Ca2+ (0.9%)	Muscle contraction, second messenger

Hormonal Control of Calcium Balance

• Parathyroid hormone (PTH):

  • Released from parathyroid glands when plasma Ca2+ is low (sensed by CaSR).

  • Raises plasma Ca2+ by mobilizing bone calcium, enhancing renal reabsorption, and indirectly increasing intestinal absorption.

• Calcitriol (Vitamin D3):

  • Enhances Ca2+ uptake in intestines, renal reabsorption, and mobilization from bone.

  • Production regulated by PTH.

• Calcitonin:

  • Produced by thyroid C cells; released when plasma Ca2+ is high.

  • Decreases bone resorption and increases renal excretion.

  • Used to treat Paget's disease.

Summary Table: Endocrine Control of Calcium Balance

Hormone	Source	Main Actions
PTH	Parathyroid glands	Raises plasma Ca2+ (bone resorption, renal reabsorption, intestinal absorption)
Calcitriol	Kidney (from Vitamin D3)	Enhances Ca2+ uptake and reabsorption
Calcitonin	Thyroid C cells	Lowers plasma Ca2+ (decreases bone resorption, increases excretion)

Additional info:

• Bone remodeling is a dynamic process involving osteoblasts (bone formation) and osteoclasts (bone resorption).

• Disorders such as osteoporosis result from excessive bone resorption relative to deposition.