Class 12 Endocrinology and Hormone of Thyroid Gland NEET

Endocrinology and Hormone of Thyroid Gland

1. Thyroxine(T4)     

2. Triiodothyronine (T3 )

Both are Iodinated form of amino acid tyrosine.

3. Thyrocalcitonin –

secreted from the parafollicular cell.

l      Thyroxine was first studied by Kendall.(1914)

l      It was isolated by E.C. Kocher.

l      It’s molecular structure was given by Harrington and Barger.

l      Thyroxine contains 65% iodine by weight.

l      95% of the iodine in our body is stored and concentrated in the thyroid gland.

l      The most active form of thyroxine is triiodothyronine (T3).

Formation of thyroxine

l      Thyroxine is made up of iodine & tyrosine. Formation of one molecule of thyroxine involves 2 molecules of tyrosine & 4 atoms of iodine. The tyrosine is 35% & the iodine is 65% of the thyroxine.

l      Follicular cell absorbs iodine from food.

l      Inorganic iodine in the presence of peroxidase enzyme converted into organic iodine.

l      Organic iodine combined with tyrosine amino acid forms thyronine.

l      MIT and DIT are unstable while T3 and T4 are stable forms of thyronine.

l      In the blood, thyroxine is found in the form of 20% T3 & 80% T4. T3 is more powerful than T4.

l      T4 is a stable form of thyroxine. The T4 converts into T3 in the target cell to perform a function. Hence, T3 is the functional form of thyroxine.

l      In blood thyroxine is transported with the help of plasma protein globulin.

l      Per day 1mg thyroxine is required for the body.

l      IRMA(Immuno radiometric assay)is the test for thyroxine.

Function of Thyroid

1.   It controls the BMR (Basal Metabolic Rate) of the body.

2.   It promotes body growth.

3.   It mainly promotes the development of the nervous system and reproductive system.

4.   It promotes tissue differentiation – by this metamorphosis in tadpole is promoted.

5.   It controls the differentiation of gonads.

6.   The thyroid increases catabolism which produces energy and increases body temperature. This process is called Calorigenic Effect. Hence, its hormones are also called as a calorigenic hormone.

Deficiency/Disorder of Thyroid :

l      The disorder related to the thyroid gland can be due to its abnormal secretion in the body of a child or adult. It may be :

       (a) Hypothyroidism                       (b) Hyperthyroidism

l      The various diseases caused due to deficiency /excess are as follows-       

       (i)    Deficiency of thyroxine in children causes cretinism (childhood hypothyroidism) and chiles called creatine.


       l       Retarded mental & physical growth.

       l       Sterile & no sexual maturity.

       l       Thick tongue, lips & skin.

       (ii)   Deficiency of thyroxine in adults initially causes simple goitre. There is swelling of the throat which is due to an enlarged thyroid. It is normally caused due to deficiency of iodine in the drinking water. At high altitudes, the goitre is very common. These areas are called a goitre area.

       (iii)  Table salt is useful to prevent goitre.

       (iv) Prolonged deficiency in adults causes myxedema which is also called as Gull’s disease. (adult hypothyroidism)


       l       Decreased BMR, heartbeat, B.P.

       l       Increased obesity and oedema.

       l       Loss of body hair.

       l       Skin becomes rough, dry and cool.

       l       My body temperature is low.

       (iv) Excess of thyroxine in both children & adult causes Exophthalamic goitre or Gravie’s disease.


       l       Swelling of the throat

       l       Bulging of eyeballs due to accumulation of interstitial fluid.

       l       Increased BMR and heartbeat

       l       Increased BP & temperature

       l       General weakness.

Hashimoto’s Disease :

l      It is an autoimmune disease of the thyroid.

l      Hyposecretion of thyroxine

l      It is also called suicide of thyroid.


l      Seafood and onion are good sources of iodine.

l      Iodine is added to table salt (NaCl) for iodine supplement.

l      Goitre BeltThe area where people suffer from goitre is called Goiter belt, due to deficiency of iodine in food, these people suffer enlarged thyroid gland known as Goitre.

l      It ranges from the Himalayas to Gorakhpur in the U.P.

l      Mexican axolotls are amphibians which ordinarily exists in aquatic larval form without being metamorphosized into adult form. It is so neoteny (Sexual maturity in the larval form) They are born deficient in the thyroid.

l      But during the administration of thyroid hormones they metamorphose into a terrestrial form.


l      Hormone secreted by Parafollicular ‘C’ cells.

l      It controls calcium and phosphorus metabolism.

l      Calcitonin is secreted when the plasma Ca2+ rises.

l      It lowers both calcium and phosphorus level in blood by reducing their mobilisation from bone.

l      It retards bone dissolution and checks osteoporosis.

l      This is antagonistic to parathormone.

2.   Parathyroid Gland.

       l      These glands remain embedded in the dorsal surface of Thyroid Gland.

       l      They are two pairs in number.

       l      Parathyroid gland is present in all the vertebrates except fishes.

       l      Its measures about 5x3x2 mm and is1.5g in weight.

       l      Parathyroids contain two types of cells.

       l      Chief cells – secrete PTH or Collip’s hormones.

       l      Oxyphil cells -Supporting cells.           

       l      Parathyroid gland secretes parathormone, which was discovered by collip. Hence, it is also called as Collip’shormone . which is under the feedback control of blood Ca2+ level.

       l      The increase in blood Ca++ (Hypercalcemia) & decreases PO4. Accordingly, it increases absorption of Ca++, increases excretion of PO4 and dissolves out Ca++ from the bones.

       l      It increases the concentration of calcium in blood plasma.

       l      It mobilises more Ca2+ from bone to blood plasma and reduces urinary elimination of calcium.

       l It mobilises more Ca2+ from bone to blood plasma and reduces urinary elimination of calcium.

       l, On the other hand, it decreases phosphatic level by increasing its elimination in the urine.            

l Parathoarmone causes Hypercalcemia and Hypophosphatemia.