Endocrinology

Endocrinology

Overview of Endocrine System

  • Cell-to-cell signaling: communication process between cells.

  • Chemical messengers: hormones or hormone-like substances secreted from endocrine glands or nerve endings to target cells.

Types of Chemical Messengers

  1. Endocrine Messengers

    • Synthesized by endocrine glands; transported via blood.

    • Example: Growth hormone, insulin.

  2. Paracrine Messengers

    • Diffuse from control cells to target cells through interstitial fluid.

    • Example: local hormones.

  3. Autocrine Messengers

    • Affect the same cells that secrete them.

    • Known as intracellular chemical mediators.

    • Example: leukotrienes.

  4. Neurocrine Messengers

    • Neurotransmitters and neurohormones.

Neurotransmitters and Neurohormones

  • Neurotransmitter: endogenous signaling molecules transmitting information between nerve cells or to muscles.

    • Examples: Acetylcholine, Dopamine.

  • Neurohormone: chemical released by nerve cells into the blood affecting distant target cells.

    • Examples: Oxytocin, ADH.

Hormones

  • Half-life of Hormones:

    • Time for a hormone's activity to decrease by half.

    • Represents the duration a hormone remains active in circulation.

Study of Endocrine Disorders

  • Causes: hyperactivity or hypoactivity of endocrine glands.

  • Signs: detected physical features during examination.

  • Symptoms: noticeable changes experienced by patients.

  • Syndrome: combination of signs and symptoms indicating a potential disease.

Types of Hormones

  1. Steroid Hormones

    • Derived from cholesterol; secreted by adrenal cortex, gonads, and placenta.

  2. Protein Hormones

    • Peptides from pituitary gland, parathyroid glands, pancreas, and placenta.

  3. Thyroid Hormones

    • Derived from tyrosine; includes T3, T4.

Hormonal Action

  • Hormones combine with specific receptors, forming hormone-receptor complexes that direct activity in target cells.

  • Receptor Locations:

    • Protein hormones: cell membrane receptors.

    • Steroid hormones: cytoplasmic receptors.

    • Thyroid hormones: nuclear receptors.

Regulation of Hormone Receptors

  • Down Regulation: decrease in receptor numbers due to hormone excess.

  • Up Regulation: increase in receptor numbers due to hormone deficiency.

Fate of Hormone Receptors

  • Internalization: hormone-receptor complexes entered by endocytosis; some receptors are recycled. Hormone level increases lead to decreased receptor numbers.

Mechanism of Hormonal Action

  1. Altering permeability of cell membranes.

  2. Activating intracellular enzymes.

  3. Acting on genes.

By Altering Permeability of Cell Membrane

  • Example: Acetylcholine increases sodium permeability at neuromuscular junctions.

By Activating Intracellular Enzymes

  • Protein hormones and catecholamines activate intracellular enzymes.

    • First messenger (the hormone) binds to receptor; forms hormone-receptor complex.

    • Second messenger: cyclic AMP (cAMP) executes hormonal effects.

Actions of cAMP

  • Stimulates enzymes for hormonal actions; varies by target cell function.

By Acting on Genes

  • Thyroid and steroid hormones create hormone-receptor complexes that bind to DNA, enhancing transcription of mRNA, ultimately producing proteins for responses in target cells.

Pituitary Gland

  • Known as Hypophysis; located in the sphenoid bone base of the skull; connected to the hypothalamus.

  • Divisions:

    1. Anterior Pituitary (Adenohypophysis)

    2. Posterior Pituitary (Neurohypophysis)

Development of Pituitary Gland

  • Anterior pituitary arises from pharyngeal epithelium; posterior pituitary arises from the hypothalamus.

Hypothalamo-hypophyseal Relationship

  • Hormones from hypothalamus reach anterior pituitary via portal blood vessels.

  • Hormones are transported to posterior pituitary via nerve fibers.

Anterior Pituitary Functions

  • Regulates other endocrine glands; composed of three parts: pars distalis, pars tuberalis, and pars intermedia.

  • Cell Types:

    • Chromophobe Cells: precursors to secretory cells.

    • Chromophil Cells:

      • Somatotrophs: secretion of growth hormone.

      • Corticotrophs: secretion of ACTH.

      • Thyrotrophs: secretion of TSH.

      • Gonadotrophs: secretion of FSH and LH.

      • Lactotrophs: secretion of prolactin.

Regulation of Anterior Pituitary Secretion

  • Hypothalamus: controls anterior pituitary via releasing and inhibitory hormones transported through portal vessels.

Hormones Secreted by Hypothalamus

  • GHRH: stimulates GH release.

  • GHIH: inhibits GH release.

  • TRH: stimulates TSH release.

  • CRH: stimulates ACTH release.

  • GnRH: stimulates FSH and LH release.

Growth Hormone (GH)

  • Source: Somatotrophs.

  • Normal values: Adult up to 300 ng/dL, children up to 500 ng/dL.

  • Actions:

    • Growth of body; regulates metabolism of proteins, fats, carbohydrates.

    • Hypersecretion causes gigantism; deficiency in children leads to dwarfism.

Regulation of GH Secretion

  • Stimulating factors: hypoglycemia, fasting, trauma, stress.

  • Inhibiting factors: hyperglycemia, increased blood fatty acids.

Disorders of Pituitary Gland

  1. Gigantism: Excess GH before epiphyseal fusion; characterized by large stature.

  2. Acromegaly: Excess GH after epiphyseal fusion; indicated by enlargement of extremities and face.

  3. Cushing’s Disease: Obesity caused by adenoma of anterior pituitary increasing ACTH, stimulating cortisol release.

  4. Dwarfism: Hyposecretion of GH during infancy.

  5. Acromicria: Atrophy of extremities due to deficiency of GH in adults.

  6. Simmonds’ Disease: Rapid senile decay due to hypopituitarism.

Disorders Related to Hypothalamus and Posterior Pituitary

  1. SIADH: Excessive ADH causing hyponatremia and fluid retention.

  2. Diabetes Insipidus: Excessive water excretion due to ADH deficiency, characterized by polyuria and polydipsia.

Thyroid Gland Hormones

  • T3, T4 (thyroxine) and Calcitonin.

  • Functions:

    • Increase basal metabolic rate (BMR); stimulate growth in children.

    • Regulate metabolism of proteins, fats, and carbohydrates.

Thyroid Disorders

  1. Hyperthyroidism: Excess TSH leading to elevated levels of hormones.

  2. Hypothyroidism: Deficiency of thyroid hormones causing myxedema in adults and cretinism in children.

  3. Goiter: Enlarged thyroid gland caused by iodine deficiency.

Parathyroid Glands

  • Secrete parathormone regulating blood calcium and phosphate levels.

  • Disorders include Hypoparathyroidism and Hyperparathyroidism, affecting calcium metabolism.

Pineal Gland

  • Secretes melatonin, influencing sleep and regulating reproductive hormones by inhibiting the onset of puberty.

Thymus

  • Functions in T lymphocyte processing and secretes thymosin and thymopoietin.

Kidney Functions

  • Secretions include erythropoietin, thrombopoietin, renin, calcitriol, and prostaglandins, all involved in various metabolic functions.

Adrenal Gland Hormones

  • Corticosteroids: mineralocorticoids, glucocorticoids, sex hormones.

    • Mineralocorticoids: aldosterone regulating sodium and potassium levels.

    • Glucocorticoids: cortisol affecting glucose metabolism and stress response.

    • Sex hormones: mainly androgens, influencing secondary sexual characteristics.

Diabetes Mellitus

  • Metabolic disorder characterized by high blood glucose due to insulin deficiency.

    • Type 1: autoimmune destruction of beta cells.

    • Type 2: insulin resistance often related to obesity.

Diagnostic Tests for Diabetes

  • Fasting blood glucose tests, postprandial blood glucose levels, and Hemoglobin A1c tests.