Class 12 Endocrinology Notes NEET AIIMS


  • Coordination in the body of all the higher vertebrates is done by two systems namely the Nervous system and endocrine system.
  • Nervous and endocrine systems are called an integrated system of the body.
  • The endocrine system achieves coordination and integration by transmitting information through Chemical Messengers called hormones.
  • These hormones are secreted by endocrine glands.
  • The branch & biology dealing with the study of the endocrine gland and its physiology is called endocrinology.
  • Thomas Addison is known as the father of endocrinology.
  • The word hormone was introduced by Starling.
  • First discovered hormone was secretin. It was discovered by Bayliss and Starling.
  • The term hormone was given as secretin (released from duodenum).
  • First experimental work on endocrinology was done by Berthold.
  • Endocrine glands and their  hormones  can be studied by : 
  • Surgical method.                     

By giving extract of the gland.

  • By use of radioisotopes – e.g. I131 isotope is used to detect cancer of the thyroid.                     
  • (iv) Radioimmunoassay – By this method trace amount of hormone are measured in the blood.
  • Hormones regulate metabolism, growth, secretion, digestion, excretion, reproduction etc.
  • Hormones are also known as ‘Autocoids’.
  • The glands can be mainly classified into two types –          
  • Exocrine Glands (with duct) 
  • Endocrine Gland (without duct)
  • Exocrine glands secrete enzymes which act as biocatalysts.
  • The endocrine gland secretes hormones which are the chemical messenger.
  • Two other types of glands are also present:-

(a)  Heterocrine Gland or Mixed Gland These glands have both types of endocrine and exocrine portion eg. pancreas.       

(b) Mixed Organ Both endocrine and metabolic or cytogenic function. eg. Gonads.

Properties of hormones

(a) Hormones are secreted by the endocrine gland (biogenic in origin).

(b) Their secretions are released directly into the blood (except local hormones e.g., gastrin).

(c) These are carried to distantly located specific organs, called the target organ.

(d) These have specific physiological actions (excitatory or inhibitory) to maintain homeostasis.

(e) The hormones have a low molecular weight e.g., ADH has a molecular weight of 600–2000 daltons.

(f) These act in very low concentration e.g. oligodynamic in nature.

(g) Hormones are non-antigenic.

(h) They destroy after the action. So it has no cumulative effect.

(i)   Rate of action of hormones is different, some are quick acting e.g., adrenalin, and some are slow-acting e.g., oestrogen of the ovary.

(j) Some hormones are secreted in an inactive form, they are called Prohormone e.g., Pro-insulin.

(k) Hormones are not stored in the gland except-thyroxin.

(l)   Hormones are work specific and target specific. Only those target cells respond to a particular for which they have receptors.

 (m)  Hormones are chemical messengers which are secreted from one part of the body and are poured directly into the bloodstream.

(n)  They reach the target place with the help of blood.

Origin of Endocrine glands

Endocrine Glands

Thyroid gland

l      It is the largest endocrine gland in the adult stage.

       It is endodermal in origin.

l      It measures 5x3x3 cm and  is25 gm in weight.

l      It is located just below pharynx at the fourth ring of trachea.

l      In human female thyroid gland is larger than the male. Most thyroid disorders are found in females.

l      It is bilobed in mammals and birds and single lobed in reptiles.

l        Thyroid is homologous to endostyle of urochordate.

l      The two lobes are interconnected by a transverse non-glandular band called Isthmus.

l      Thyroid is composed of spherical sacs called thyroid follicles, made of cuboidal epithelium

l      Follicular cell secretes colloid substances called thyroglobulin. (not hormones)

l     The thyroid is stored in the lumen of the follicle.

l      Between thyroid follicles, small bunches of the cell are present called parafollicular cells, or C-cells.  

C-cells are remnants of the ultimobranchial gland of fish.