Endocrine System Overview

Learning Objectives

• Review the physiology of hormones produced by the pituitary, thyroid, and adrenal glands.
• Learn laboratory tests for diagnosing common endocrine disorders.
• Interpret test results for diagnosis and patient management.
• Understand stimulation and suppression tests for specific endocrine disorders.
• Apply knowledge for rational use of endocrine tests.

Endocrine System Outline

• Introduction to the endocrine system
• Pituitary function
• Thyroid function
• Adrenal function

The Endocrine System

• Endocrine function: secreting hormones into the bloodstream.
  • Hormones act at distant sites.
  • Autocrine: act on themselves.
  • Paracrine: act on adjacent cells.
  • Intracrine: act within the cells.
• Control metabolism, growth, reproduction, electrolytes balance, and stress responses.
• Endocrine hormones: released by glands/cells into blood, affecting target cells.
• Neuroendocrine hormones: secreted by neurons into blood and affect target cells.

Pituitary Gland

• Hypophysis: located in sella turcica, connected to hypothalamus.
• Divided into anterior (adenohypophysis) and posterior (neurohypophysis) lobes.
• Anterior lobe produces trophic hormones:
  • Growth hormone (GH): stimulates protein synthesis and growth.
  • Thyroid-stimulating hormone (TSH): stimulates thyroid hormone production.
  • Adrenocorticotropic hormone (ACTH): stimulates adrenal cortex hormones.
  • Prolactin (PRL): stimulates milk production.
  • Follicle-stimulating hormone (FSH): affects ovarian follicles/spermatogenesis.
  • Luteinizing hormone (LH): stimulates ovulation, testosterone production.
• Posterior lobe stores/releases hormones made in hypothalamus:
  • Antidiuretic hormone (ADH): increases water reabsorption, vasoconstriction.
  • Oxytocin: stimulates milk ejection, uterine contractions.

Chemical Structure and Synthesis of Hormones

• Proteins/polypeptides: most common, >100 amino acids (proteins) <100 (peptides).
• Steroid hormones: derived from cholesterol, lipid-soluble, rings structure.
• Amine hormones: derived from tyrosine - thyroid and adrenal medullary hormones.

Thyroid Gland

• Located below the larynx, two lobes connected by an isthmus.
• Produces thyroid hormones (T3 and T4) and calcitonin.
• T3 and T4 increase metabolic rate, T3 produced by other tissues.
• Calcitonin regulates calcium homeostasis.

Thyroid Function Tests

• Thyroid-stimulating hormone (TSH): measures thyroid function, most useful.
• Total T3 and T4: measure total serum levels of T3 and T4, useful but free T3/T4 preferred.
• Measure free T4 and free T3.
• Thyroid function tests help diagnose disorders like hypothyroidism, hyperthyroidism, etc.

Regulation of ADH (Antidiuretic Hormone)

• Factors promoting ADH secretion: increased sodium concentration, decreased blood volume/pressure.
• Factors inhibiting ADH secretion: decreased sodium concentration, increased blood volume/pressure, alcohol.

Syndrome of Inappropriate ADH Secretion (SIADH)

• SIADH: high ADH levels causing hyponatremia (low sodium) and hyperosmolar urine.
• Causes: include CNS diseases, tumors, pulmonary infections, and medications.

Diabetes Insipidus (DI)

• DI: inability of the kidneys to concentrate urine; high urine output and elevated blood osmolarity.
• Central DI: insufficient ADH production.
• Nephrogenic DI: kidney resistance to ADH.
• Diagnosed by water deprivation tests.

Pituitary Tumors

• Microadenomas (<1 cm) and macroadenomas (≥1 cm) are classified as functioning or non-functioning.
• Common functioning types: prolactinomas, somatotropinomas (GH).

Adrenal Glands and Function

• Adrenal glands: pyramidal structures above each kidney.
• Medulla (inner), cortex (outer): produce different hormones.
• Cortex (3 zones):
  • Zona glomerulosa (mineralocorticoids): aldosterone regulates sodium and water balance.
  • Zona fasciculata (glucocorticoids): cortisol for stress response, blood sugar, pressure.
  • Zona reticularis (androgens): sex hormones like DHEA.
• Medulla: produces catecholamines (epinephrine/norepinephrine).

Congenital Adrenal Hyperplasia (CAH)

• CAH: arises from errors in steroid hormone synthesis.
• 21-hydroxylase deficiency and other enzyme defects.
• Causes characteristic symptoms, depending on type.

Hypercortisolism (Cushing Syndrome)

• Excess cortisol: can be exogenous (drugs) or endogenous (due to tumors, ACTH excess).
• Clinical symptoms: striae (stretch marks), facial plethora (redness), muscle weakness, obesity.
• Tests for Cushing syndrome: urine free cortisol, overnight dexamethasone suppression test.

Adrenal Medulla

• Adrenal medulla produces catecholamines (epinephrine/norepinephrine).
• Pheochromocytoma: chromaffin tumor, producing excess catecholamines, causing hypertension.
  • Diagnosis tests include plasma-free metanephrines, 24 hr urine catecholamines, etc.

This quiz covers the essential physiology of hormones produced by the pituitary, thyroid, and adrenal glands. It includes laboratory tests for diagnosing common endocrine disorders and the interpretation of test results for effective patient management. Enhance your understanding of stimulation and suppression tests in the context of the endocrine system.