Hormones in Animals

What Are Hormones?

Hormones are chemicals secreted by the endocrine glands, which are carried by the blood to the respective organs to regulate certain physiological processes. The organs on which the hormones act are known as the target organs. The target cells have receptors on their surface that recognize the respective hormones.

Hormones in animals are secreted through two types of glands, namely:

  • Endocrine Glands– The glands that do not have ducts and transport their secretions directly to the site of action through the blood are known as endocrine glands. For eg., adrenal glands, pituitary glands, etc.
  • Exocrine Glands– These glands have ducts to pass their secretions. For eg., sweat, liver, etc.

Hormone Functions – Various Hormones And Their Functions

Glands Hormones Functions
Hypothalamus Antidiuretic hormone Regulates fluid balance inside kidneys
Pituitary Growth hormone Regulates body growth
Thyroid Thyroxine Regulates body metabolism
Pancreas Insulin and Glucagon Regulates blood sugar level
Adrenal Epinephrine Regulates heart rate and blood pressure
Testes Testosterone Development of sperms and male characteristics
Ovaries Estrogen and Progesterone Development of eggs and male characteristics


Hormones control the functions of all the organs. They affect the diverse processes of growth and development, reproduction and sexual characteristics. Very small amounts of hormones can induce very prominent responses in the body. Most of the hormones are derived from proteins.

However, too much or too less of something is always dangerous. When a hormone produces too much or too little hormones, it results in hormonal imbalance.

Types of Hormonal Imbalances

  1. Adrenal Insufficiency– Addison’s disease is caused as a result of insufficient adrenalin secretion. The symptoms like fatigue, dehydration and skin changes indicate Addison’s disease.
  2. Cushing’s Disease– Hypersecretion of pituitary gland hormone may result in an overactive adrenal gland. This is a condition similar to Cushing’s syndrome which occurs in people with high corticosteroid levels.
  3. Acromegaly (Gigantism)– The hypersecretion of growth hormone from the pituitary gland in kids results in the development of an abnormally large body.
  4. Hyperthyroidism– When the thyroid gland produces enough thyroxine hormone, it results in hyperthyroidism. The symptoms include fast heart rate, sweating, etc.
  5. Hypothyroidism– When the thyroid gland produces less amount of thyroxine, it results in hypothyroidism. The symptoms include fatigue, constipation, dry skin, etc.

Classification Of Hormones

On the basis of chemical classes/structure, hormones can be classified into the following:

  • Protein/Peptide Hormones – These hormones consist of linked amino acids. The peptide hormones are synthesized and stored in advance in the secretory vesicles. They are found in the cell membrane of the receptors and are released from the parent cell through exocytosis. Their structure is that of a polypeptide chain which is a chain of amino acids which includes shorter polypeptide chains in their molecules, namely,  oxytocin and antidiuretic hormone which are synthesized in the brain and released into the bloodstream in the posterior pituitary gland. This group also covers tiny proteins such as growth hormones that are synthesized by the pituitary and the large glycoproteins such as the FSH(follicle stimulating hormone) which is also synthesized by the pituitary. After being stored in the vesicles, a stimulus triggers a response causing it to release, for example, insulin is secreted due to high blood glucose levels. These hormones are insoluble in lipids and soluble in water. The receptors are found on the target’s cell surface as they can not permeate through the plasma membrane of the cells
  • Steroid Hormones – These hormones are derived from cholesterol, lipid-derived hormones. They are produced from precursors on demand and are released from the parent cell through simple diffusion. These hormones bind themselves to the proteins while being carried in the blood and generally have the target response of causing induction of the new protein production. These hormones are the primary group of lipids hormones. They are usually alcohols or ketones chemically. For example, Estrogen and testosterone which are secreted by the female and male reproductive system respectively.  These hormones are water-insoluble. They stay for a longer period of time in comparison to the peptide hormones as they are carried by transport proteins in the blood. Example: The hormone Cortisol. It has a half-life ranging from 60-90minutes.
  • Amino-acid derived hormones – They are comparatively smaller molecules that are derived from tryptophan and tyrosine amino acids. The norepinephrine and epinephrine are produced by the thyroid gland. Melatonin is secreted by the pineal gland situated in the brain which is responsible to control the sleep cycle.

Female Hormone List

Some hormones present in the human body are considered to be female hormones although a few of them are produced naturally in a man’s body. However, the distinguishing factor of these hormones in different bodies is the role they play in the fertility of a woman and since they are more dominant in a female’s body. Listed below are female hormones:

  • Progesterone – These hormones assist in pregnancy, prepare the uterus lining for the fertilized egg and bring down the synthesis of estrogen post the stage of ovulation
  • Estrogen – Plays a major role in puberty, pregnancy, menstruation, menopause
  • HCG(Human chorionic gonadotrophin)
  • Testosterone – Though it is considered to be a male hormone, it is similar to the amount of estrogen that is produced in men as a small amount of testosterone is also formed in women.

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