• The endocrine system consists of endocrine glands which secretes hormones.
  • Hormones are substances that are secreted by one group of cells, are secreted into the blood, and affects the physiology of another group of cells (organs). 
  • The endocrine system is controlled by the pituitary gland and the hypothalamus.
  • Endocrine organs are well vascularized.
  • A system of ductless glands
  • Endocrinology – study of hormones and endocrine glands Hormones are able to maintain homeostasis because they are subject to negative feedback mechanisms.


A hormone is a chemical transmitter. It is released in small amounts from glands and is transported in the bloodstream to target organs or other cells.  Hormones are chemical messengers,

transferring information and instructions from one set of cells to another.


Hyposecretion or hypersecretion of any hormone can be harmful to the body.  Controlling the production of hormones can treat many hormonal disorders in the body.

Hormones regulate growth, development, mood, tissue function, metabolism, and sexual


  • Hypothalamus
  • Pituitary Gland
  • Thyroid Gland
  • Parathyroid Glands
  • Thymus Gland
  • Adrenal Glands
  • Pancreas
  • Ovaries
  • Testes
  • Pineal Gland


•Part of brain

–Regulates ANS, emotions, feeding/satiety, thirst, body temperature, etc.

–Hormones related to these functions

•“Releasing hormones”

•Axonal transport to posterior lobe

Some Hypothalamus Hormones

  • Growth Hormone Releasing Hormone (GH-RH)
  • Prolactin Releasing Hormone (PRL-RH)
  • Thyroid Stimulating Hormone Releasing Hormone (TSH-RH)
  • Adrenocorticotropic Hormone Releasing Hormone (ACTH-RH)
  • Melanocyte Stimulating Hormone Releasing Hormone (MSH-RH)
  • Follicle Stimulating Hormone Releasing Hormone (FSH-RH)
  • Luteinizing Hormone Releasing Hormone (LH-RH)
  • Growth Hormone Inhibiting Hormone (GH-IH)
  • Prolactin Inhibiting Hormone (PRL-IH)
  • Thyroid Stimulating Hormone Inhibiting Hormone (TSH-IH)
  • Adrenocorticotropic Hormone Inhibiting Hormone (ACTH-IH)
  • Melanocyte Stimulating Hormone Inhibiting Hormone (MSH-IH)
  • Follicle Stimulating Hormone Inhibiting Hormone (FSH-IH)
  • Luteinizing Hormone Inhibiting Hormone (LH-IH)


The Pituitary gland, also known as the hypophysis, is a pea-sized endocrine gland situated at the base of our brain.

It is often referred to as the ‘Master Gland’ because it produces some of the important hormones in the body.

It is situated in a bony structure called the Pituitary fossa, just below the hypothalamus, close to the optic nerve. 

Secretions from the anterior pituitary gland…

  2. ACTH
  3. TSH
  4. LH & FSH
  5. PRL
  6. MSH

Growth Hormone (GH):  essential for the growth and development of bones, muscles, and other organs.

Its excess leads to gigantism and deficiency leads to dwarfism.

Adrenocorticotropin(ACTH):  essential for the growth of the adrenal cortex.

Thyroid-Stimulating Hormone (TSH):  essential for the growth and development of the thyroid gland.

Follicle-Stimulating Hormone (FSH):  is a gonadotropic hormone.

It stimulates the growth ovarian follicles in the female and the production of sperm in the male.

Prolactin (PRL):  stimulates the development and growth of the mammary glands and milk production during pregnancy..

The sucking motion of the baby stimulates prolactin secretion.

Melanocyte-stimulating hormone (MSH): regulates skin pigmentation and promotes the deposit of melanine in the skin after exposure to sunlight

Secretion from the posterior lobe of the pituitary gland

Antidiuretic Hormone (ADH):  stimulates reabsorption of water by the renal tubules.

 Hyposecretion result in diabetes insipidus.

Oxytocin:  stimulates uterus to contract during labor

 A synthetic version of this hormone, used to induce labor, is called Pitocin.

It also stimulates the mammary glands to release milk. 


It is a pinecone-shaped small gland located in the middle of the human brain in between the two hemispheres in an area called epithalamus.

It was once known as “the third eye”. It is the major site for melatonin secretion, which regulates the body’s internal clock (Circadian rhythm)


The pineal gland is pine-cone-shaped and only about 1 cm in diameter

Melatonin: Has some effect on sleep/awake cycles and other biological clock

Serotonin: a neurotransmitter that regulates intestinal movements and affects appetite, mood, sleep, anger, and metabolism.


The thyroid gland is a ductless endocrine gland situated in the anterior/front portion of the neck.

It roughly resembles the shape of a butterfly.

It is also one of the largest endocrine glands, weighing an average of 25 – 30 g.

This gland has two lobes on either side of the trachea, with each lobe measuring 4 – 6 cm in length and 1.3 – 1.8 cm in width.


Largest endocrine gland in body

Yellow brown in color

Highly vascularised

Situated in front of neck at level of c5-c7 vertebra

It has 2 lobes connected by an isthmus

Both the lobes are conical in shape

Weight of gland 30gms

Capsule- true capsule-condensation of CT

False capsule-pretracheal layer of deep cervical fascia

Blood supply– superior throid artery

Inferior thyroid artery

Venous drainage- superior inferior and middle thyroid veins

Produces two hormones

Thyroid hormone (TH)



•Thyroid hormone (TH)

–Acts on most cells of the body

–Increases metabolic rate

–Iodine is needed to make TH

•Calcitonin Lowers blood calcium


The functional unit of the thyroid gland is the thyroid follicle. The cells are called follicular cells and secrete the light purple liquid within the follicle, called colloid. Colloid is water, filled with a lot of protein called thyroglobulin..

TSH is what stimulates the follicular cells to make thyroglobulin.  TSH also increases the size of the follicle to accommodate all this protein.

The thyroid gland plays a vital role in metabolism and regulates the body’s metabolic processes.

Calcitonin:  influences bone and calcium metabolism;  maintains a homeostasis of calcium in the blood plasma 

T3 & T4

Thyroxine (T4) and triodothyronine (T3):  essential for basal metabolic rate

Influences physical/mental development and growth.

Hyposecretion of T3 and T4 = cretinism,  myxedema, Hashimoto’s disease

Hypersecretion of T3 and T4 = Grave’s disease, goiter, Basedow’s disease


The parathyroid glands are small endocrine glands situated just below the thyroid glands in the neck.

They are usually four in number, two behind each thyroid gland.

They are very small, pea-sized and weigh about 50 mg. The glands function to maintain the calcium and phosphorus levels in our bodies.


The two pairs of parathyroid glands are located on back side of thyroid gland.

Secrete parathormone (PTH) which plays a role in the metabolism of phosphorus.

 Too little results in cramping; 

too much results in osteoporosis or kidney stones.


Pancreas is an abdominal organ located behind the stomach and surrounded by spleen, liver and small intestine. It is a vital part of the digestive system and is responsible for regulating blood sugar levels.

•Is an endocrine and exocrine gland

–Exocrine cells – acinar cells – secrete digestive enzymes into a duct.

–Endocrine cells – pancreatic islets (islets of Langerhans)

•Secrete insulin

•Secretes glucogon


The islets of Langerhans are small clusters of cells located in the pancreas.


Alpha cells- secrete glucagon

Facilitate breakdown of glycogen to glucose.

To leveates blood sugar.

Beta cells secrete insulin Essential for maintenance of normal blood sugar levels.  Inadequate levels result in diabetes mellitus.

Delta cells suppress the release of glucagon and insulin.

7. The Adrenal Glands

•Located on the superior surface of kidneys


Secretes catecholamines (mostly epinephrine)

–Active in “fight, flight, and fright” response


Secretes aldosterone (salt and water balance for blood pressure)

Secretes androgens and estrogens (sex hormones)

Secretes cortisol (anti-stress and anti-inflammation hormone)


The triangular-shaped adrenal glands are located on the top of each kidney. The inside is called the medulla and the outside layer is called the cortex.


Cortisol:  regulates carbohydrate, protein, and fat metabolism;  has an anti-inflammatory effect;  helps the body cope during times of stress.

Hyposecretion results in Addison’s disease;  hypersecretion results in Cushing’s disease.

Corticosterone:  like cortisol, it is a steroid;  influences potassium and sodium metabolism

Aldosterone:  essential in regulating electrolyte and water balance by promoting sodium and chloride retention and potassium excretion.Androgens:  several hormones including testosterone;  they promote the development of secondary sex characteristics in the male.


Epinephrine is also called adrenalin.  It elevates systolic blood pressure, increases heart rate and cardiac output, speeds up the release of glucose from the liver… giving a spurt of energy, dilates the bronchial tubes and relaxes airways, and dilates the pupils to see more clearly. It is often used to counteract an allergic reaction.

Norepinephrine, like epinephrine, is released when the body is under stress. It creates the underlying influence in the fight or flight response. As a drug, however, it actually triggers a drop in heart rate.

8. ovary

The ovary is a primary gonad of the female reproductive system. It produces the female reproductive cells called the ovum (non-motile).

The ovum is fertilised by a motile sperm to form a zygote. Also, the ovaries act as an endocrine gland by releasing certain hormones in females

Secretions from the ovaries…

Ovaries produce estrogen & progesterone

 These hormones prepare the uterus for pregnancy, promote the development of mammary glands, play role in sex drive, develop secondary sex characteristics in female.

Estrogen is essential for the growth, development, and maintenance of female sex organs.

The ovary is a primary gonad of the female reproductive system. It produces the female reproductive cells called the ovum (non-motile). The ovum is fertilised by a motile sperm to form a zygote. Also, the ovaries act as an endocrine gland by releasing certain hormones in females.


testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testosterone.

Secretions of the testes…

The testes produce the male sex hormone called testosterone.  It is essential for normal growth and development of the male sex organs. Testosterone is responsible for the erection of the penis.

Secretions of the placenta…

During pregnancy, the placenta serves as an endocrine gland. 

It produces chorionic gonadotropin hormone, estrogen, and progesterone.

Secretions of the thymus…


The thymus gland has two lobes, and is part of the lymphatic system.  It is a ductless gland, and secretes thymosin. This is necessary for the Thymus’ normal production of T cells for the immune system.

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