Human Endocrine System and Hormone Functions

Posted on Jun 24, 2026 in Medicine

The Endocrine System

The endocrine system is a collection of glands that produce hormones to regulate many physiological functions in the body, such as growth, metabolism, and sexual function.

What are Glands?

Glands are specialized organs or tissues that produce and secrete substances such as hormones, enzymes, or other body fluids to regulate various bodily functions. They are classified into two types:

  • Exocrine Glands: These glands release substances through a duct or canal to a specific location. They secrete enzymes and other substances that assist with digestion, absorption, and other bodily functions.
  • Endocrine Glands: These are ductless glands that release hormones directly into the bloodstream to regulate metabolic processes, growth, and development.

Major Endocrine Glands

The various endocrine glands include:

  • Pituitary Gland
  • Thyroid Gland
  • Adrenal Glands
  • Pancreas
  • Pineal Gland
  • Parathyroid Glands
  • Thymus Gland
  • Gonads (Testes in males / Ovaries in females)
  • Hypothalamus

Endocrine Glands Diagram Summary

The system includes the Pineal gland, Hypothalamus, Pituitary glands, Thyroid gland, Parathyroid glands, Thymus gland, Adrenal gland, Pancreatic islets, Ovaries (females), and Testes (males).

The endocrine system works together with the nervous system to control and coordinate body functions. This integrated network is known as the neuroendocrine system, and its study is called neuroendocrinology.

Understanding Hormones

Hormones are chemical messengers produced by glands (specialized tissues or cells) that transmit messages from one cell to another. They reach their target sites through blood circulation.

The main function of hormones is to control and coordinate the functions of different tissues and organs while assisting in growth and development.

Flowchart: Glands → Hormones → [Bloodstream] → Target Cells → Functions

Properties of Hormones

  • They may be peptides, steroids, amines, or derivatives of amino acids.
  • They act in very low concentrations.
  • They produce their effects on specific target cells.
  • They are rapidly destroyed after their function is complete.

Classification of Hormones

Based on chemical composition, hormones are categorized into two types:

1. Lipid-Soluble Hormones

These are lipid-soluble and include:

  • Steroid hormones: Derived from cholesterol (e.g., estrogen, testosterone).
  • Thyroid hormones: Produced by the thyroid glands (e.g., T3 and T4).
  • Nitric oxide (NO).

2. Water-Soluble Hormones

These are water-soluble and include:

  • Amine hormones: (e.g., Epinephrine, Norepinephrine).
  • Peptide hormones: (e.g., Insulin, Growth hormones, ACTH, TSH).
  • Protein hormones: (e.g., Luteinizing hormone (LH), FSH).

Mechanism of Hormone Action

Hormones travel through the blood to reach their target sites and perform functions by binding to receptors located inside or on the surface of cells. They perform their action by forming a hormone-receptor complex.

General Mechanism Flow: Hypothalamus → Glands → Hormone → Receptors on Cells → Action/Response.

1. Lipid-Soluble Hormone Mechanism

The lipid-soluble hormone reaches the target cell and diffuses into the cell. It binds with a receptor to form a hormone-receptor complex, which alters gene expression. This forms new mRNA, which synthesizes proteins, leading to altered cell activity.

2. Water-Soluble Hormone Mechanism

Water-soluble hormones reach the target site through the blood and bind to receptors on the cell surface. This activates a secondary messenger (such as cAMP, Ca²+, or cGMP), which activates protein kinase enzymes. This causes phosphorylation, forming phosphorylated proteins that trigger physiological actions.

The Hypothalamus

The hypothalamus is a part of the brain located at its base. Its main function is to connect the nervous system to the endocrine system through the pituitary gland.

It is considered the Master of the Master Gland because it regulates hormones by releasing various “releasing” or “inhibiting” hormones.

  • Releasing Hormones: Stimulate the release of hormones from the pituitary gland (e.g., TRH, CRH, GnRH, PRF, GHRH).
  • Inhibiting Hormones: Inhibit the secretion of pituitary hormones (e.g., Somatostatin/GRIH, PRIH).

The hypothalamus also regulates body temperature, hunger, thirst, and homeostasis.

The Pituitary Gland (Hypophysis)

Referred to as the Master Gland, it monitors and regulates many bodily functions and controls other glands. It is roughly the size of a pea (1 cm diameter, 0.5 g weight) and is located within the sella turcica at the base of the brain.

The pituitary gland is joined to the hypothalamus via the infundibulum and is divided into two lobes:

1. Anterior Pituitary (Adenohypophysis)

Comprising about 75% of the total weight, it is made of pharyngeal epithelium. It secretes:

  • Growth Hormone (GH)
  • Thyroid Stimulating Hormone (TSH)
  • Adrenocorticotropic Hormone (ACTH)
  • Prolactin
  • Gonadotropins (FSH and LH)

2. Posterior Pituitary (Neurohypophysis)

Comprising about 25% of the weight, it is made of axons and axon terminals. It secretes:

  • Oxytocin
  • Antidiuretic Hormone (ADH) or Vasopressin

Regulation of Hormones

The pituitary gland releases hormones with the help of the hypothalamus via feedback mechanisms. This involves a loop: Hypothalamus → Releasing Hormones → Pituitary Gland → Hormone Release → Blood Level Increase → Target Site Action (utilizing both positive and negative feedback loops).

Key Hormones and Their Functions

I. Growth Hormone (GH)

Also known as Somatotropic hormone, it is secreted by acidophil cells of the anterior pituitary. It is a peptide hormone that stimulates growth and cell reproduction.

  • Increases body growth and promotes utilization of fats.
  • Induces protein, fat, and carbohydrate metabolism.
  • Regulates development during childhood and adolescence.
  • Promotes cell regeneration and bone growth.
  • Involved in immune system functions.

II. Thyroid Stimulating Hormone (TSH)

A glycoprotein hormone that stimulates the production of Thyroxine (T4) and Triiodothyronine (T3) from the thyroid glands. It is regulated by Thyrotropin-releasing hormone (TRH).

III. Adrenocorticotropic Hormone (ACTH)

A peptide hormone that stimulates the production of corticosteroids (cortisol, aldosterone, and androgens) from the adrenal glands.

IV. Prolactin (PRL)

A protein hormone produced by lactotrophs that plays a crucial role in lactation. It stimulates milk production and mammary gland growth during pregnancy.

V. Gonadotropins

A group of protein hormones regulating the gonads:

  • FSH (Follicle-stimulating hormone): Stimulates follicular growth in ovaries and sperm production in testes.
  • LH (Luteinizing hormone): Triggers ovulation and testosterone production.

VI. Oxytocin

Produced by the hypothalamus and released by the posterior pituitary, it is often called the “love hormone.” It stimulates uterine contractions during childbirth and regulates milk ejection during lactation.

VII. Anti-Diuretic Hormone (ADH)

Also known as Vasopressin, it regulates water balance and blood pressure by decreasing urine excretion and increasing water absorption by the kidneys.

The Adrenal Glands

Also known as Suprarenal glands, these are located on top of the kidneys. They play a vital role in stress response, blood pressure regulation, and electrolyte balance.

Anatomical Layers (Cortex to Medulla)

  • Outermost: Zona glomerulosa
  • Middle: Zona fasciculata
  • Inner: Zona reticularis

I. Adrenal Cortex

The outermost layer (80-90% of the gland) contains three zones:

  • Zona Glomerulosa: Secretes mineralocorticoids (Aldosterone).
  • Zona Fasciculata: Secretes glucocorticoids (Cortisol).
  • Zona Reticularis: Secretes sex hormones (Androgens).

Mineralocorticoids

Aldosterone stimulates sodium resorption and potassium excretion. Along with angiotensin II, it stimulates the RAAS system to regulate blood pressure.

Glucocorticoids

Cortisol is the main glucocorticoid. It is essential for life, increasing blood sugar levels, producing anti-inflammatory responses, and regulating the immune system.

Sex Hormones

Androgens help in the development of sexual characteristics during puberty.

II. Adrenal Medulla

The inner part (10-20% of the gland) is made of nervous tissue and is part of the sympathetic nervous system. It secretes:

  • Epinephrine (Adrenaline): Prepares the body for “fight or flight” by increasing heart rate and energy metabolism.
  • Norepinephrine (Nor-adrenaline): Contributes to stress response and attention.

The Thyroid Gland

The largest endocrine gland, located in the neck in front of the larynx. It is H-shaped (butterfly-shaped) and weighs about 25g. It consists of thyroid follicles and parafollicular cells.

Thyroid Hormones (T3 and T4)

Tri-iodothyronine (T3) and Thyroxine (T4) are made from iodine. They are essential for growth, skeletal development, and regulating the Basal Metabolic Rate (BMR).

Calcitonin

Also known as thyrocalcitonin (TCT), it is released from parafollicular cells to regulate calcium levels in the blood by inhibiting bone resorption.

The Pancreas

A composite organ acting as both an exocrine (95%) and endocrine (5%) gland. It is located posterior to the stomach.

Endocrine Pancreas (Islets of Langerhans)

  • Alpha cells: Secrete Glucagon.
  • Beta cells: Secrete Insulin.
  • Delta cells: Secrete Somatostatin.

Glucagon

A peptide hormone that raises blood glucose levels by stimulating glycogenolysis and gluconeogenesis.

Insulin

Regulates blood glucose by facilitating glucose uptake in cells, promoting glycogen synthesis, and preventing diabetes mellitus.

Somatostatin

Inhibits the secretion of both insulin and glucagon, as well as Growth Hormone from the pituitary.

The Pineal Gland

The smallest endocrine gland, located in the diencephalic area. It secretes melatonin, which regulates the circadian rhythm (sleep-wake pattern) and the release of sex hormones.

The Thymus Gland

A lobular structure located in the thoracic cavity. It plays a major role in the immune system by secreting thymosin, which promotes the maturation of T-cells (lymphocytes) and the production of antibodies.