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Human Endocrine System

Endocrine System and Hormones
The endocrine system consists of a group of specialized glands called endocrine glands and the substances called hormones, which are secreted by them. The endocrine glands do not have a duct system; hence the hormones are directly secreted into the bloodstream. Therefore, they are also called ductless glands. Hormones are chemical substances which catalyse and control various metabolic processes. The hormones such as thyroxin, adrenal steroids, sex hormones and insulin of vertebrates come under this class of chemical substances.

Hormones are diverse in chemical nature and biological properties. Chemically, hormones may be polypeptides, steroids, amino acid derivatives or biogenic amines. Protein hormones or peptide hormones are produced by the hypothalamus, pituitary, parathyroids and pancreas. Relaxin is a protein hormone produced from the ovary and a hormone from the placenta (human chorionic gonadotropin) is also another protein hormone. Steroid hormones are produced from the adrenal cortex, ovary and testes. Biogenic amines are produced by the adrenal medulla and the pineal gland. The hormone of the thyroid gland called thyroxin is an iodinated derivative of an amino acid.

The various endocrine glands and their location in the body are shown in the figure. The functions of various endocrine glands like the thyroid, adrenal, ovary and testes are regulated by hormones from the pituitary gland. The functions of the pituitary gland, in turn, are regulated by the hormones of the hypothalamus. The primary target of the hypothalamic hormones is the pituitary gland. Other endocrine glands which are influenced by the pituitary gland may be considered as secondary targets of the hypothalamic hormones.

Hormones reaching their target organs through the blood stimulate the synthesis of specific proteins or other secretory products in the target cells. The hormones, first, are recognized at specific recognition sites known as hormone receptors. These receptors are present either on the cell membrane or in the cytoplasm and nucleus of the cell. The synthesis of specific proteins leads to other biological, biochemical and physiological actions of individual hormones. The location, structure and functions of the major endocrine glands are given below:

Location of various endocrine glands in the body of the human being

Pineal Gland

It is found in the midbrain, at the roof of the diencephalon. It produces several hormones. One of the hormones called melatonin may delay sexual development and may reduce sexual activity. Hence it is antigonadal in function. In Amphibia, melatonin makes the skin paler. Melatonin is a nonprotein hormone and is synthesised from another amine called serotonin. Serotonin is a vasoconstrictor and helps to decrease the diameter of blood vessels.

It is a sac like gland situated on the ventral side of the brain (diencephalon region). The pituitary gland consists of three parts: anterior, intermediate and posterior lobes. Each of these parts of the pituitary secretes various hormones which are all proteins or polypeptides.
  1. Anterior Lobe
    The anterior lobe of the pituitary secretes the following hormones:
    1. Somatotropic or Growth Hormone (STH/GH)
      It controls growth. A deficiency of this hormone produces dwarfism. An excess of this hormone produces gigantism in children and acromegaly (abnormal growth of bones of face, lower jaw, hands and feet) in adults.
    2. Thyrotropic or Thyroid Stimulating Hormone (TSH)
      As the name implies it stimulates secretion of thyroxine from the thyroid gland.
    3. Adreno - Corticotropic Hormone (ACTH)
      It stimulates the adrenal cortex thereby regulating the secretion of steroids from the adrenal glands.
    4. Lactogenic Hormone or Prolactin
      It stimulates the mammary glands of the female to produce milk after childbirth.
    5. Follicle Stimulating Hormone (FSH)
      It stimulates the ovaries and controls the development of Graafian follicles in which ova develop. It also controls the secretion of steroids by the ovaries. In males it is essential for spermatogenesis.
    6. Luteinising Hormone (LH)
      It brings about the luteinisation of the post ovulatory follicle. It brings about ovulation and controls the secretion of ovarian steroids.

      In the male, LH controls the secretion of androgens in the testes. It also regulates spermatogenesis. Since it stimulates the interstitial cells in the testes to produces the male sex hormone, it is also called interstitial cell stimulating hormone or ICSH.
  2. Intermediate Lobe
    The intermediate lobe of the pituitary produces only one protein hormone called melanocyte stimulating hormone or MSH which makes the skin colour darker in lower vertebrates like amphibia.
  3. Posterior Lobe
    The posterior pituitary releases two peptide hormones into the blood:
    1. Oxytocin
      It causes the contraction of the uterus in the female during childbirth and also helps in the expulsion of milk from the breast during suckling of the child.
    2. Antidiuretic hormone (ADH) or Vasopressin
      ADH increases water re absorption in the distal convoluted tubules and collecting tubules of the kidney and thus cuts down the excess loss of water in the urine. Deficiency of ADH secretion increases urine flow to a large extent (diabetes insipidus).

Thyroid Gland

It is a bilobed gland lying beneath the larynx. It consists of a mass of rounded vesicles held together by connective tissue and interwoven with blood capillaries. Each vesicle is lined by an epithelial layer enclosing a colloidal substance.

The thyroid gland secretes a hormone called thyroxine which contains large amounts of iodine. The hormone performs the following functions:
  1. It regulates the growth of the cells of the body.
  2. It regulates the rate of oxidation of food and thus, helps in releasing energy during metabolism.
  3. Its deficiency (hypothyroidism) causes cretinism which is characterised by stunted growth, bodily deformity, defective mental development and retarded sexual development in children. Hypothyroidism also results in myxoedema in adults.  This results in loss of hair, increased thickness and dryness of skin, increase in weight, muscular weakness, slowing of mental process and diminution of metabolism.
  4. The excess of thyroxine (hyperthyroidism) causes a disease called exophthalmic goitre in which there is rapid heart beat, fast pulse rate, restlessness and nervousness, loss of weight and protruding eye-balls. An enlarged thyroid in the adult results in a swelling in the neck. This disease is called simple goitre.
  5. Calcitonin is another hormone from the thyroid. This protein hormone reduces the blood calcium level.


There are four small rounded structures situated on the dorsal side of the thyroid, two on each side. Parathyroids secretes a hormone called parathormone (PTH) which is a protein. It increases the blood calcium level by metabolizing calcium from bone to blood and also by lowering urinary excretion of calcium and this increases the excitability of nerves and muscles abnormally (parathyroid tetany). Excessive secretion of parathormone leads to a fall in bone minerals leading to softening, bending and fracture of bones, increased levels of blood calcium and decreased urinary excretion of calcium leading to calcification of soft tissues like blood vessels. (Osterititis fibrosa cystica). Thus parathormone mainly influences calcium metabolism; parathyroids may also secrete some calcitonin.

It is a large, somewhat pinkish, lymphoid mass situated behind the sternum. It secretes thymosin, a protein hormone which probably stimulates the proper differentiation of lymphocytes in the foetus and the newborn child.

It is a lobulated gland, which lies embedded in the mesentery connecting the two limbs of the duodenum. It consists of several lobules which are held together by connective tissue. The lobules secrete pancreatic juice which is digestive in function. Among the lobules lie groups of cells called Islets of Langerhans.

These islands of cells constitute the endocrine part of the pancreas. The β cells of the islets secrete the hormone insulin and the α cells secrete glucagon. Both these hormones are proteins.

Insulin regulates the amount of sugar in the blood and controls the metabolism of carbohydrates in the body. The deficiency of insulin causes a condition called diabetes mellitus. Normally insulin lowers the blood sugar level by increasing the utilisation of glucose in the tissue cells and the storage of glucose as glycogen in the tissues. If the pancreas does not secrete sufficient amounts of insulin, the liver and muscles are unable to convert the excess blood glucose into glycogen and glucose utilisation for energy production or fat production is also reduced; so more and more glucose accumulates in the blood (hyperglycemia). The excess of glucose is eliminated through urine (glucosuria). This condition is called diabetes mellitus. Injection of insulin to diabetic patients lowers the blood sugar. Excessive secretion of insulin leads to an abnormal lowering of blood glucose (hypoglycemia).

Glucagon increases the conversion of liver glycogen to glucose (glycogenolysis) and this glucose enters the blood to raise the blood sugar level. Thus the effect of glucagon is opposite to the effect of insulin.

Adrenal Gland
Adrenal glands are a pair of small rounded bodies lying one on top of each kidney. Each gland has two regions:
  1. Cortex
    It is the outer part of the gland. This part of the adrenal gland secretes a number of steroid hormones. Glucocorticoids are steroid hormones from the adrenal cortex, which influence metabolism of carbohydrates, e.g. cortisol and cortisone. They increase the blood glucose level by increasing formation of carbohydrates from protein (gluconeogenesis) and also by conversion of glycogen to glucose in the liver (glycogenolysis). The steroid hormones which influence salt and water metabolism are called mineralocoticoids, e.g. aldosterone. They reduce the loss of sodium from the body through urine, sweat etc., and increase the excretion of potassium. These hormones help in the retention of water by the body along with sodium and hence, maintain salt and water balance. Some other steroid hormones from the adrenal cortex influence the external sex characters, these are called sex corticoids.
  2. Medulla
    It is the inner, central part of the adrenal gland. The medulla of each adrenal secretes two amines: epinephrine (adrenalin) and nor epinephrine (noradrenalin). These hormones regulate the blood pressure, heart beat, respiration, contraction and relaxation of smooth muscles and other activities controlled by the sympathetic nervous system. Epinephrine also increases levels of blood glucose, oxygen consumption and heat production. During stress, epinephrine is secreted and it produces increased blood pressure, rapid and stronger heart beats and release of more sugar from the liver to the blood (glycogenolysis) to provide more material for increased oxidation and production of energy.

Renin-Angiotensin System

Near the glomeruli in the cortex of the kidney are certain cells called juxtaglomerular cells. These cells are believed to secrete a protein called renin under the influence of nervous stimulation and other factors. It helps to change the plasma protein, angiotensinogen to angiotension. Angiotensin stimulates the adrenal cortex to secrete a hormone called aldosterone. Angiotensin also causes vaso-constriction to raise the blood pressure. Angiotensinogen is produced in the liver.

Gonads-Ovary and Testes

The gonads, ovaries in the female and testes in the male, apart from producing ova and spermatozoa, also function as endocrine glands. These produce what are known as sex hormones.
  1. Ovary
    In females, a pair of ovaries - one on each side of the lower part of the abdomen - constitutes the gonads. Ovaries secrete female sex hormones, oestradiol and progesterone. Both of these are steroids chemically. At puberty, these hormones regulate the growth and development of secondary female sex organs like the uterus and mammary glands and of female accessory sex characters like deposition of fat on thighs and the female pattern of pubic hair distribution. They also maintain the normal functions of the secondary sex organs and control their changes during menstrual cycle and pregnancy. Another hormone from the ovary is relaxin which is a peptide hormone. This hormone softens the pelvic ligament so that childbirth can take place easily.
  2. Testes
    In the male, the two testes located in the scrotal sac between the thighs are the gonads. Testes secrete male sex hormones called androgens of which testosterone is the principal one. The androgens are all steroids. At puberty, testosterone stimulates the growth and development of male secondary sex organs like prostrate and seminal vesicles and also male accessory sex characters like deepening of voice, broadening of shoulders and the male pattern of distribution of pubic hair. It also maintains the male secondary sex organs and their functions in the adult male.


Until recently, it was considered that the pituitary gland was the chief gland of the endocrine system. It is now known that the pituitary itself is under the control of the hypothalamus of the brain. The pituitary gland is connected to the hypothalamus by the pituitary stalk. The anterior pituitary is connected to the hypothalamus by nerve fibres. The function of the anterior pituitary is regulated by hormones (neuro hormones) secreted by the neuro secretory cells in the hypothalamus. Oxytocin and vasopressin are actually produced in the hypothalamus and are only stored in the posterior pituitary.

Gastrointestinal Hormones
Gastrointestinal (G) hormones are secreted into the blood by specialized cells lining the gastrointestinal tract, whose aggregate mass is greater than that of the rest of the endocrine system.

Defects or injury to the hypothalamus, the pituitary, or the testes or ovary, result in hypogonadism. Male hypogonadism can consist of deficient androgen production (hypofunction of Leydig cells), deficient sperm formation (hypo function of Sertoli cell) or both, before puberty. It results in the lack of development of secondary sexual characteristics and male musculature.

Female hypogonadism results from hyposecretion of estrogen, resulting in cessation of reproductive cycles. Such hypogonadism can result from a shortage of pituitary gonadotropins (LH, FSH, or both) or can represent primary testicular/ovarian failure.


This results from the failure of testosterone secretion. For this disorder, secondary sex organs, such as prostate gland, seminal vesicles and penis, remain infantile and small in size and fail to function. Spermatozoa fail to be produced. Secondary sexual characteristics like beards, moustaches and low pitch male voice fail to develop.

It is the development of breast tissue in males, and is usually due to perturbation of estrogen to androgen ratio. In the neonatal period and during puberty, gynaecomastia is due to temporary increase in circulating estrogen. Decreased testosterone in later life may also lead to gynaecomastia.

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