What is Thyroid Gland? Structure & Function

The thy­roid gland is high­ly vas­cu­lar gland. Sur­round­ed by a fibrous cap­sule. It con­sist of two lobes lying on either side if the tra­chea. The lobes are joined togeth­er by an isth­mus lying in front of tra­chea. The lobes are joined togeth­er by an isth­mus lying in front of tra­chea. The lobes are con­i­cal in shape about 5 cm long and 3cm wide and have upper and low­er poles. It receives plen­ti­ful blood sup­ply from supe­ri­or thy­roid artery which is branch from the exter­nal carotid artery and inte­ri­or carotid a branch from the sub­cla­vian artery. The parathy­roid glands lie in the pos­te­ri­or sur­face of each lobe and are embed­ded in the thy­roid tissue.

Thyroid Gland
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Structure of Thyroid Gland 

Micro­scop­i­cal­ly the thy­roid glands con­tain two types of hor­mone pro­duc­ing cells.

  1. The Fol­lic­u­lar cells which pro­duce thy­roid hor­mones i.e. thy­rox­in and Tri idothyronine 
  2. The C or clear cells which pro­duces thyrocalcitonin.

The struc­ture units of the thy­roid glands are small sacs or bags known as fol­li­cle between the fol­li­cles there us a net­work of retic­u­lar fibrous, cap­il­lar­ies, lym­phat­ic and nerves. Each fol­li­cle are formed by a sin­gle lay­er of epithe­lial cells/Follicular cells. That sur­round­ed by a cav­i­ty which con­tains a secre­to­ry prod­uct known as col­loid. The Col­loid is a vis­cous homo­ge­neous flu­id, con­sist­ing main­ly of a pro­tein Iodine com­plex called hemo­glo­bin which is pro­duce d by the fol­lic­u­lar cells. The col­loid also con­tains the type thy­roid­hor­mones in the stor­age from and is made up of pro­teins, thy­roid hor­mones. The tri­iodothy­ro­nine (T3) and Thy­rox­in (T4). When these hor­mones are need­ed by the body an enzyme (pro­tein split­ting enzymes) is the col­loid splits the thy­ro­glo­bin mol­e­cules are tak­en up by the fol­lic­u­lar cells for entry into the blood steam.

On enter­ing the blood stream, thy­rox­in and Tri­iodothy­ro­nine (T 3 and T 4). Com­bined with a glo­b­ing frac­tions of plas­ma pro­tein and are trans­port­ed to all parts of the body as ’ pro­tein — bound’ iodine com­pounds. Iodine is essen­tial for the for­ma­tion of thy­roid hor­mone T 3 and T 4 both the thy­roid hor­mone con­tains a high pro­mo­tion of iodine which the thy­roid gland is obtained from food espe­cial­ly sea food and water an iodine defi­cien­cy will lead into enlarge­ment of the thy­roid gland to form a goiter.

Control of Secretion

Thy­roid stim­u­lat­ing hor­mone from ante­ri­or pitu­itary con­trols the growth and sec­ondary activ­i­ty of the thy­roid gland. It increas­es the size and no of fol­lic­u­lar cells . TSH also pro­mote the break­down of thy­ro­glo­bin in the fol­li­cles with the resul­tant release of tri­iodothy­ro­nine an thyroxin.

The secre­tion of thy­roid stim­u­lat­ing hor­mones by the ante­ri­or pitu­itary is reg­u­lat­ed by the con­cen­tra­tion of tri­iodothy­ro­nine or Thy­rox­in in the cir­cu­lat­ing blood. When the con­cen­tra­tion of this hor­mone is decreased the pitu­itary cells are stim­u­lat­ed by a way hypo­thal­a­mus to secre­tion .TSH on the hand and increase in the amount of these hor­mones in the blood leads . to inhib­it the pro­duc­tion of thy­roid stim­u­lat­ing hormones.

Functions of Thyroid Gland

  1. Tri iodothy­ro­nine and Thy­rox­in con­trols the meta­bol­ic rate of the body.
  2. It rais­es the O2 con­sump­tion and increase the heat production.
  3. They are essen­tial for nor­mal growth and devel­op­ment of the body includ­ing men­tal devel­op­ment and attain­ment of sex­u­al maturation.
  4. They have under spread effects on all aspects of metab­o­lism such as pro­mot­ing pro­tein syn­the­sis main­taing nor­mal blood lev­els of cho­les­terol and fat­ty acid increas­ing the rate of glu­cose . Absorp­tion and utilisation.
  5. They assist in main­taing flu­id and elec­trolyte balance .
  6. It also affects the irri­tabil­i­ty of cen­tral ner­vous system.
  7. If plays a part in keep­ing the skin and hair in a good condition
  8. Stor­age of iodine.

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