Hormones in Humans: Definition, Types and Functions

Have you ever wondered what hormones are and how they influence our everyday lives? Hormones are crucial chemical messengers that regulate countless functions in our bodies, from growth and development to mood and metabolism. In this article, we will define hormones, explore the types of hormones, delve into the function of hormones, discuss the classification of hormones, and learn the characteristics of hormones. By the end, you will clearly understand what hormones are in the human body and discover answers to important questions like What is called hormones.

What Are Hormones?

To define hormones simply, they are chemical substances secreted by endocrine glands (ductless glands) directly into the bloodstream. They travel to specific target organs or cells, where they trigger or regulate physiological processes. Understanding what hormones in the human body help explain how our bodies maintain balance (homeostasis) and respond to changes in our environment.

For instance, hormones can be explained as special chemical signals made in certain glands. These signals are then released into the bloodstream and travel to different parts of the body to control various activities like growth, digestion, and metabolism. 

Characteristics of Hormones

To understand the characteristics of hormones, note that they:

1. Are produced in trace amounts by specific glands.

2. Act on target cells or organs with specific receptors.

3. Have a short lifespan in the bloodstream, ensuring precise control.

4. Regulate a wide range of functions, including growth, metabolism, reproduction, and more.

5. Work on a feedback system, meaning their secretion is often regulated by either positive or negative feedback loops.

Classification of Hormones

The classification of hormones typically depends on their chemical makeup and how they interact with cells. Broadly, they can be classified into two major categories:

1. Peptide Hormones

• Made up of chains of amino acids.

• Generally water-soluble and cannot easily pass through the cell membrane.

• Require specific receptors on the cell surface to initiate a response inside the cell.

• Example: Insulin.

2. Steroid Hormones

• Derived from cholesterol.

• Fat-soluble and can pass directly through the cell membrane.

• Bind to receptors inside the target cell.

• Example: Testosterone, Oestrogen.

Types of Hormones

When we refer to the types of hormones, we are usually talking about the major groups based on their source glands or specific functions. Here are a few key hormones:

1. Growth Hormone (GH)

• Secreted by the pituitary gland.

• Stimulates growth and cell reproduction.

2. Thyroxine (T4) and Triiodothyronine (T3)

• Produced by the thyroid gland.

• Regulate metabolism, heart rate, and body temperature.

3. Insulin

• Secreted by the pancreas.

• Controls blood sugar levels by allowing cells to absorb glucose.

4. Adrenaline (Epinephrine)

• Produced by the adrenal glands.

• Prepares the body for ‘fight or flight’ by increasing heart rate and blood flow to muscles.

5. Testosterone

• Secreted by the testes in males and smaller amounts by the ovaries in females.

• Essential for male secondary sexual characteristics, muscle mass, and bone density.

6. Oestrogen and Progesterone

• Produced by the ovaries.

• Vital for female secondary sexual characteristics, menstrual cycle regulation, and pregnancy maintenance.

Endocrine Glands and Their Hormones

The human body contains several endocrine glands that each release specific hormones:

• Hypothalamus: Controls body temperature, hunger, thirst, and the release of hormones from the pituitary gland.

• Pituitary Gland: Called the “master gland” because it secretes hormones that regulate other glands, including growth hormone, thyroid-stimulating hormone (TSH), and more.

• Pineal Gland: Produces melatonin, which regulates sleep patterns.

• Thyroid Gland: Secretes T3 and T4, influencing metabolism and energy levels.

• Parathyroid Gland: Regulates calcium levels in the blood.

• Thymus: Important for T-cell development in the immune system.

• Adrenal Glands: Produce adrenaline (epinephrine), cortisol, and aldosterone (helps balance salt and water in the body).

• Pancreas: Releases insulin and glucagon, playing a crucial role in blood sugar regulation.

• Testes: Produce testosterone, essential for male characteristics and reproduction.

• Ovaries: Produce oestrogen and progesterone, crucial for female reproductive health.

Functions of Hormones in the Human Body

One of the main reasons we need to understand what hormones are in the human body is because they are integral to maintaining homeostasis. The functions of hormones include:

1. Regulating Growth and Development: Growth hormone and thyroid hormones influence height, weight, and overall body structure.

2. Controlling Metabolism: Hormones like insulin, glucagon, and thyroid hormones help manage energy production, storage, and expenditure.

3. Managing Stress: Adrenal hormones (cortisol and adrenaline) modulate the body’s response to stress by increasing alertness and energy availability.

4. Reproduction: Sex hormones control puberty, menstrual cycles, sperm production, and pregnancy.

5. Maintaining Homeostasis: Hormones ensure stable internal conditions such as temperature, blood sugar levels, and fluid balance.

Diseases Caused by Hormonal Imbalances

Even small fluctuations in hormone levels can significantly affect health. Common disorders include:

• Diabetes Mellitus: Caused by insufficient insulin production or ineffective insulin action, leading to high blood sugar.

• Hyperthyroidism: Overactive thyroid gland causing rapid metabolism, weight loss, and restlessness.

• Hypothyroidism: Underactive thyroid gland leading to weight gain, fatigue, and slow metabolism.

• Osteoporosis: Often linked to decreased oestrogen or testosterone, affecting bone density.

• Acromegaly: Excess growth hormone in adulthood, leading to enlarged bones in the hands, feet, and face.

• Gigantism: Excess growth hormone in childhood, resulting in abnormal height and growth.

Feedback Mechanisms in Hormone Regulation

Hormones often work through feedback mechanisms:

• Positive Feedback: Increases the output of a process. For instance, during childbirth, the hormone oxytocin intensifies contractions.

• Negative Feedback: Most hormone regulation uses negative feedback. For example, when thyroxine levels are too high, the hypothalamus reduces the release of Thyrotropin-Releasing Hormone (TRH), which in turn lowers Thyroid-Stimulating Hormone (TSH) secretion, normalising thyroxine levels.

Unique Ways to Remember Hormones (Mnemonics)

Here is a simple mnemonic to recall some key endocrine glands and their primary hormones: “HPTA-POP”

• H: Hypothalamus – Regulates pituitary hormone release

• P: Pituitary – “Master gland” (Growth hormone, TSH)

• T: Thyroid – T3, T4

• A: Adrenal – Adrenaline, cortisol

• P: Pancreas – Insulin, glucagon

• O: Ovaries – Oestrogen, progesterone

• P: Pineal – Melatonin

This mnemonic helps you remember the primary endocrine glands and the hormones they secrete.

Short Quiz on Hormones

Test your knowledge with these questions:

1. Which gland is referred to as the “master gland”?
   A. Thyroid
   B. Pituitary
   C. Adrenal
   D. Pancreas

2. Which hormone is responsible for regulating the sleep-wake cycle?
   A. Insulin
   B. Thyroxine
   C. Melatonin
   D. Testosterone

3. Which hormone controls blood sugar levels in the body?
   A. Insulin
   B. Oestrogen
   C. Adrenaline
   D. Thyroxine

4. Which disorder is characterised by an underactive thyroid gland?
   A. Hyperthyroidism
   B. Hypothyroidism
   C. Diabetes
   D. Acromegaly

5. Which hormones are primarily associated with female reproductive health?
   A. Insulin and Glucagon
   B. Oestrogen and Progesterone
   C. Testosterone and Melatonin
   D. Adrenaline and Cortisol

(Answers: 1-B, 2-C, 3-A, 4-B, 5-B)