What Are Hormones? Types of Hormones, How They Work?

  • The term hormone was coined by “Starling“.
  • The first discovered hormone is Secretin. It was discovered by “Bayliss & Starling in 1902″.
  • Hormones are also called “Primary messengers” or “Chemical messengers“.

Classical definition – Hormones are chemicals produced by endocrine glands and released into the blood and transported to distantly placed target organs.

Modern scientific definition – Hormones are non-nutrient chemicals that act as an intercellular messenger and are produced in trace amounts.

Facts about hormones

  • Produced in very small quantities because these are very reactive substances.
  • Hormone molecules are small, and their molecular weight is low.
  • Most of the hormones are water-soluble but some are lipid-soluble and are easily diffusible in tissues.
  • They can not be stored in the body (Except thyroxine).
  • Hormones are destroyed after use i.e, hormones can not be reused in the body.
  • The liver and kidneys separate hormones from blood and decompose them.
  • The product formed after decomposition is excreted with urine.
  • Hormones are non-antigenic & non-species-specific substances.
  • Usually, hormones do not participate in the metabolic activities of target cells but they affect and control the activity level of these target cells.

Mechanism of hormone action

  • Once hormone enters the bloodstream it can reach almost up to any cell in the body. However, each hormone affects only a certain kind of cells, which are called target cells.
  • All hormones do not act in the same way due to the different locations of their receptors. Fat-soluble hormones have receptors inside the cell but water-soluble hormones have receptors on the cell because the permeability of the plasma membrane is different for water and fat.
  • Hormone receptors present on the cell membrane of the target cells are called membrane-bound receptors and the receptors present inside the target cell are called intracellular receptors, mostly nuclear receptors (present in the nucleus).
  • The binding of a hormone to its receptor leads to the formation of a hormone-receptor complex.
  • Each receptor is specific to one hormone only and hence receptors are specific. We can also say hormones are receptor-specific.
  • Hormone-Receptor complex formation leads to certain biochemical changes in the target tissue. Thus, target tissue metabolism and physiological functions are regulated by hormones.

Nature of hormone

  1. Water-soluble hormones
  2. Lipid or fat-soluble hormones

Water-soluble hormones

Water-soluble hormones interact with membrane-bound receptors and normally do not enter into the target cell, but generate secondary messenger e.g. cyclic AMP, IP3, Ca+2, etc.) which in turn regulate cellular metabolism.

Example: Proteinaceous hormones

Lipid or fat-soluble hormones

Lipid or fat-soluble hormones interact with intracellular receptors (mostly nuclear).

  • Mostly regulate gene expression or chromosome function by the interaction of the hormone-receptor complex with the genome. Cumulative biochemical action results in physiological and developmental effects.
  • The action of lipid-soluble hormones is slower and long-lasting than the action of water-soluble hormones. 

Example: Steroidal hormones, Iodothyronine, etc.

List of hormones with their nature and source gland

Nature of HormoneHormoneSource Gland
1. Proteinaceous
(i) Amino acid derivatives
Thyroxine or Iodothyronine
Epinephrine
Norepinephrine (NE)
Melatonin
Thyroid gland
Adrenal medulla
Adrenal medulla
Pineal gland
(ii) Short PeptidesVasopressin
Oxytocin
Melanocyte Stimulating Hormone (MSH)
Hypothalamus
Hypothalamus
Adenohypophysis
(iii) Long PeptidesParathyroid hormone
Insulin
Thyrocalcitonin or Calcitonin
ACTH
Parathyroid
Pancreas
Thyroid
Adenohypophysis
(iv) GlycoproteinThyroglobulin
FSH
LH
TSH
Thyroid
Adenohypophysis
Adenohypophysis
Adenohypophysis
2. SteroidsMineralocorticoids (Aldosterone)
Glucocorticoid (Cortisol)
Testosterone
Progesterone
Oestrogen or estrogen
Adrenal cortex
Adrenal cortex
Testes
Ovary
Ovary
3. Fatty – Acid DerivativesProstaglandin (PG)Local cells
List of hormones their nature and source gland

Types of hormone

Hormones mainly are of two types-

  1. Synergistic hormones
  2. Antagonistic hormones

Synergistic hormones

When two or more hormones complement the function of each other and both are needed for the full expression of hormonal effects is called synergistic hormones. These effects cannot be observed by a single hormone alone. Thus, the synergistic effect produces greater effects than the sum of the individual effects of the hormones.

Example: Insulin and growth hormone/thyroxin have a synergistic effect on the physical growth of the mammary gland.

Antagonistic hormones

Antagonistic hormones are a pair of hormones in which two hormones oppose the actions of each other.

Example: Insulin and glucagon (insulin functions to decrease the blood glucose levels, whereas glucagon functions to increase blood glucose levels), parathyroid hormone and calcitonin, MSH, and melatonin

Know more from endocrinology

Chemical control and coordinationParathyroid Gland their hormones and disorders
Pituitary Gland – Its Hormones, functions, and disordersHypothalamus and Hypothalamic hormones
Thyroid Gland and its hormonesComplete Endocrinology
Chemical control and coordination

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