Endocrine System: Hormones, Glands, and Disorders – Study Guide

Chapter 16: Endocrine System

Overview of Hormones

The endocrine system regulates physiological processes through the secretion of hormones. Hormones are chemical messengers that travel through the bloodstream to target organs, influencing growth, metabolism, and homeostasis.

• Amino-acid based hormones: Derived from amino acids, these hormones are generally water-soluble and bind to receptors on the cell membrane. Examples include insulin, growth hormone, and thyroid-stimulating hormone (TSH).

• Lipid-based hormones: Mostly steroid hormones derived from cholesterol, these are lipid-soluble and bind to intracellular receptors. Examples include cortisol, aldosterone, and sex hormones (estrogen, testosterone).

• Receptor location: Amino-acid based hormones bind to membrane receptors; lipid-based hormones bind to cytoplasmic or nuclear receptors.

• Effects: Hormones can alter gene expression, enzyme activity, or membrane permeability.

Endocrine vs. Exocrine Glands

Glands are classified based on how they release their products.

• Endocrine glands: Secrete hormones directly into the bloodstream (e.g., pituitary, thyroid).

• Exocrine glands: Release substances through ducts to the outside of the body or into the digestive tract (e.g., sweat glands, salivary glands).

Regulation of Hormone Release

Hormone secretion is tightly regulated to maintain homeostasis.

• Negative feedback: Most hormones are regulated by negative feedback, where increased hormone levels inhibit further secretion.

• Positive feedback: Less common; hormone release is amplified (e.g., oxytocin during childbirth).

Hypothalamic Control of the Endocrine System

The hypothalamus is a key regulator of the endocrine system, controlling the pituitary gland and thus many other endocrine glands.

• Three main mechanisms:

  1. Secretion of regulatory hormones to control anterior pituitary

  2. Production of hormones stored and released by the posterior pituitary

  3. Neural control over the adrenal medulla

Major Endocrine Organs

• Infundibulum: Connects hypothalamus to pituitary gland.

• Neurohypophysis: Posterior pituitary, stores and releases hormones produced by the hypothalamus.

• Adenohypophysis: Anterior pituitary, produces and releases its own hormones.

Hypothalamic-Pituitary Axis

This axis is central to endocrine regulation, integrating signals from the brain and controlling hormone release throughout the body.

• Hypophyseal portal system: Network of blood vessels connecting hypothalamus and anterior pituitary, allowing regulatory hormones to reach target cells efficiently.

• Purpose: Rapid and direct communication for precise hormonal control.

Tropic vs. Nontropic Hormones

Hormones are classified based on their target and function.

• Tropic hormones: Stimulate other endocrine glands to release hormones (e.g., TSH, ACTH, FSH, LH).

• Nontropic hormones: Directly affect target tissues (e.g., growth hormone, prolactin).

Thyroid Hormones: Synthesis and Function

The thyroid gland produces hormones essential for metabolism and development.

• Thyroid hormones (T3, T4): Synthesized from tyrosine and iodine in the thyroid gland.

• Thyroglobulin: Precursor protein for thyroid hormone synthesis.

• Importance of iodine: Required for hormone production; deficiency leads to disorders like goiter.

Thyroid Tissue and Hormone Secretion

• Follicular cells: Secrete T3 and T4.

• Parafollicular cells: Secrete calcitonin, which lowers blood calcium levels.

Adrenal Gland Structure and Function

The adrenal glands consist of two main regions, each producing distinct hormones.

• Adrenal cortex: Produces corticosteroids.

  • Zona glomerulosa: Mineralocorticoids (e.g., aldosterone)

  • Zona fasciculata: Glucocorticoids (e.g., cortisol)

  • Zona reticularis: Gonadocorticoids (e.g., androgens)

• Adrenal medulla: Produces catecholamines (epinephrine, norepinephrine)

Pancreatic Hormones and Glucose Regulation

The pancreas regulates blood glucose through the secretion of insulin and glucagon.

• Islets of Langerhans: Clusters of endocrine cells in the pancreas.

• Alpha cells: Secrete glucagon, which increases blood glucose.

• Beta cells: Secrete insulin, which decreases blood glucose.

• Mechanism: Insulin promotes glucose uptake by cells; glucagon stimulates glycogen breakdown and gluconeogenesis.

Glycogenolysis and Gluconeogenesis

• Glycogenolysis: Breakdown of glycogen to glucose.

• Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources.

Parathyroid Hormone (PTH)

• Parathyroid glands: Located on the posterior surface of the thyroid.

• PTH: Increases blood calcium by stimulating bone resorption, increasing intestinal absorption, and reducing renal excretion.

Major Hormones: Synthesis, Release, and Effects

Each hormone has a specific site of synthesis, stimulus for release, target tissue, and physiological effect.

Hormone	Synthesized In	Stimulus for Release	Target Tissue	Physiological Effect
ADH (vasopressin)	Hypothalamus (released by posterior pituitary)	High blood osmolarity	Kidneys	Water reabsorption, increased blood pressure
GH	Anterior pituitary	GHRH from hypothalamus	Most tissues	Growth, protein synthesis
TSH	Anterior pituitary	TRH from hypothalamus	Thyroid gland	Stimulates thyroid hormone release
ACTH	Anterior pituitary	CRH from hypothalamus	Adrenal cortex	Stimulates corticosteroid release
FSH, LH	Anterior pituitary	GnRH from hypothalamus	Gonads	Regulate reproductive processes
Insulin	Pancreatic beta cells	High blood glucose	Most tissues	Decreases blood glucose
Glucagon	Pancreatic alpha cells	Low blood glucose	Liver	Increases blood glucose
Cortisol	Adrenal cortex	ACTH	Most tissues	Stress response, increases blood glucose
Aldosterone	Adrenal cortex	Low blood sodium, high potassium	Kidneys	Increases sodium reabsorption
Estrogen, Testosterone	Gonads	FSH, LH	Reproductive tissues	Sexual development, reproduction
Leptin	Adipose tissue	Fat storage	Hypothalamus	Regulates appetite
Melatonin	Pineal gland	Darkness	Brain	Regulates sleep-wake cycle

Regulatory Hypothalamic Hormones

• GHRH: Growth hormone releasing hormone

• GHIH: Growth hormone inhibiting hormone (somatostatin)

• CRH: Corticotropin releasing hormone

• TRH: Thyrotropin releasing hormone

• GnRH: Gonadotropin releasing hormone

• PIH: Prolactin inhibiting hormone

Endocrine Disorders

Hormonal imbalances can lead to various diseases and syndromes.

• Diabetes mellitus Type I: Autoimmune destruction of beta cells; insulin deficiency.

• Diabetes mellitus Type II: Insulin resistance; often associated with obesity.

• Goiter: Enlargement of the thyroid gland, often due to iodine deficiency.

• Hypothyroidism: Low thyroid hormone levels; symptoms include fatigue, weight gain.

• Hyperthyroidism: Excess thyroid hormone; symptoms include weight loss, increased metabolism.

• Cretinism: Congenital hypothyroidism; leads to developmental delays.

• Myxedema: Severe hypothyroidism in adults.

• Metabolic calcification: Abnormal calcium deposition due to hormonal imbalance.

Key Equations

• Blood glucose regulation: $\text{Glucose}_{\text{blood}} = \text{Glucose}_{\text{intake}} - \text{Glucose}_{\text{utilization}}$

• Hormone feedback loop (simplified): $\text{Stimulus} \rightarrow \text{Endocrine gland} \rightarrow \text{Hormone release} \rightarrow \text{Target tissue response} \rightarrow \text{Feedback to gland}$

Additional info: Some details, such as the full list of hormones and their effects, were expanded for completeness and academic clarity.