Endocrine System: Major Glands, Hormones, and Mechanisms (Study Guide)

Endocrine System Overview

Introduction

The endocrine system is a network of glands that produce and secrete hormones to regulate various bodily functions. Understanding the anatomy, histology, and physiology of these glands is essential for comprehending how the body maintains homeostasis.

Major Endocrine Glands

Pituitary Gland

The pituitary gland, often called the "master gland," is located at the base of the brain and consists of two major lobes: the anterior (adenohypophysis) and posterior (neurohypophysis) lobes.

• Major Lobes: Anterior (adenohypophysis) and posterior (neurohypophysis).

• Development: The anterior lobe develops from the mouth (Rathke's pouch), while the posterior lobe develops from the hypothalamus.

• Hormones Produced:

  • Anterior pituitary: Growth hormone (GH), prolactin (PRL), adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH).

  • Posterior pituitary: Antidiuretic hormone (ADH), oxytocin.

• Receptors: Hormone receptors are located on target organs and tissues, mediating specific physiological effects.

• Portal System: The hypophyseal portal system transports hormones from the hypothalamus to the anterior pituitary.

• Diseases: Disorders include pituitary adenomas, hypopituitarism, and diabetes insipidus.

Additional info: The posterior pituitary does not have a portal system because it releases hormones directly into the bloodstream.

Thyroid Gland

The thyroid gland is located in the anterior neck, below the larynx, and consists of two lobes connected by an isthmus. It is organized histologically into follicles filled with colloid.

• Major Hormones: Thyroxine (T4) and triiodothyronine (T3). T3 is more biologically active; T4 is produced in greater abundance.

• Synthesis: Hormones are synthesized in the thyroid follicles. Iodine is essential for hormone formation.

• Key Molecules: Thyroglobulin, DIT (diiodotyrosine), MIT (monoiodotyrosine).

• Receptors: Located in target tissues; thyroid hormones act via nuclear receptors.

• Actions: Increase metabolic rate, regulate growth and development.

• Regulation: Controlled by TSH from the anterior pituitary and negative feedback mechanisms.

• Disorders: Hyperthyroidism (e.g., Graves' disease), hypothyroidism (e.g., Hashimoto's thyroiditis).

• Other Hormones: Calcitonin, which lowers blood calcium levels.

Parathyroid Glands

Parathyroid glands are small glands located on the posterior surface of the thyroid gland. Most people have four parathyroid glands.

• Hormone Produced: Parathyroid hormone (PTH).

• Function: Regulates blood calcium levels by increasing calcium reabsorption in the kidneys, releasing calcium from bones, and increasing calcium absorption in the gut.

• Regulation: Controlled by negative feedback based on blood calcium levels.

Adrenal Glands

The adrenal glands are located above the kidneys and consist of two major parts: the cortex and the medulla.

• Other Name: Suprarenal glands.

• Anatomical Parts: Cortex (outer) and medulla (inner).

• Development: Cortex and medulla develop from different embryological origins.

• Cortex Layers:

  • Zona glomerulosa: Produces mineralocorticoids (e.g., aldosterone).

  • Zona fasciculata: Produces glucocorticoids (e.g., cortisol).

  • Zona reticularis: Produces androgens.

• Medulla: Produces catecholamines (epinephrine and norepinephrine).

• Regulation: Cortex is regulated by ACTH; medulla by sympathetic nervous system.

• Diseases: Cushing's syndrome (hypersecretion), Addison's disease (hyposecretion).

Pineal Gland

The pineal gland is a small endocrine gland located in the brain. It produces melatonin.

• Hormone Produced: Melatonin.

• Function: Regulates circadian rhythms and sleep-wake cycles.

Pancreas

The pancreas is located in the abdomen, behind the stomach, and has both endocrine and exocrine functions.

• Endocrine Cells: Islets of Langerhans (alpha, beta, delta cells).

• Major Hormones: Insulin (beta cells) and glucagon (alpha cells).

• Functions:

  • Insulin lowers blood glucose by promoting cellular uptake.

  • Glucagon raises blood glucose by stimulating glycogen breakdown.

• Diseases: Diabetes mellitus (insulin deficiency or resistance), diabetes insipidus (different etiology).

Gonads

Gonads are the primary reproductive organs: testes in males and ovaries in females.

• Major Hormones: Testosterone (males), estrogen and progesterone (females).

• Functions: Regulate reproductive development, secondary sex characteristics, and fertility.

• Regulation: Controlled by FSH and LH from the anterior pituitary.

Placenta

The placenta is a temporary endocrine organ during pregnancy.

• Hormones Produced: Human chorionic gonadotropin (hCG), progesterone, estrogen.

• Clinical Application: hCG is detected in urine for home pregnancy tests.

Other Glandular Secretions

Other hormones are produced by various tissues and organs, as described in reference tables.

• Examples: Thymus (thymosin), heart (atrial natriuretic peptide), kidneys (erythropoietin).

• Targets and Effects: Vary depending on hormone and tissue.

Hormone Signaling and Control Mechanisms

Types of Chemical Signaling

• Autocrine: Hormones act on the same cell that secreted them.

• Endocrine: Hormones travel through the bloodstream to distant targets.

• Paracrine: Hormones act on neighboring cells.

Chemical Classes of Hormones

• Amino acid-based hormones: Includes most hormones (e.g., insulin, TSH).

• Steroid hormones: Derived from cholesterol (e.g., cortisol, sex hormones).

• Eicosanoids: Derived from fatty acids (e.g., prostaglandins).

Signal Transduction Pathways

Hormones exert their effects via specific signaling pathways. The cAMP second messenger system is a common pathway for many hormones.

• cAMP Pathway: Hormone binds to receptor → activates G protein → stimulates adenylate cyclase → increases cAMP → activates protein kinase → cellular response.

Equation:

$\text{Hormone} + \text{Receptor} \rightarrow \text{G protein activation} \rightarrow \text{Adenylate cyclase} \rightarrow \text{cAMP} \rightarrow \text{Protein kinase activation} \rightarrow \text{Cellular response}$

Mechanisms of Hormone Control

• Humoral: Changes in blood levels of ions/nutrients stimulate hormone release.

• Neural: Nerve fibers stimulate hormone release.

• Hormonal: Hormones stimulate other endocrine glands to release hormones.

Summary Table: Major Endocrine Glands, Hormones, and Functions

Gland	Major Hormones	Main Functions
Pituitary (anterior)	GH, PRL, ACTH, TSH, FSH, LH	Growth, metabolism, reproduction
Pituitary (posterior)	ADH, Oxytocin	Water balance, uterine contraction
Thyroid	T3, T4, Calcitonin	Metabolism, calcium regulation
Parathyroid	PTH	Calcium regulation
Adrenal (cortex)	Aldosterone, Cortisol, Androgens	Electrolyte balance, stress response, sex hormones
Adrenal (medulla)	Epinephrine, Norepinephrine	Fight-or-flight response
Pineal	Melatonin	Sleep-wake cycles
Pancreas	Insulin, Glucagon	Blood glucose regulation
Gonads	Testosterone, Estrogen, Progesterone	Reproduction, secondary sex characteristics
Placenta	hCG, Progesterone, Estrogen	Pregnancy maintenance

Additional info: This study guide covers the major endocrine glands, their hormones, mechanisms of action, and associated diseases, providing a comprehensive overview for exam preparation in Anatomy & Physiology.