Thyroid gland :- structure and function

Thyroid gland

1. K.G. ARTS & SCIENCE COLLEGE RAIGARH (C.G.) Year - 2019-20 Deparment of zoology & research center Subject :- endocrinology Seminar topic:- thyroid gland Guided by:- Submitted by:- Prof. Ranjana sahu mam. Surendra Patel M.Sc sem I zoology

2. The thyroid gland

3. Synopsis :- Introduction History Origin Location Structure Microanatomy Thyroid hormons

4. 8 . Biochemistry of thyroid hormone. 9. Iodine and it’s uptake 10. Synthesis of thyroid hormone. 11. Transport and regulation 12. Mechanism of action 13. Function of thyroid hormonE . 14. Disorders 15. Conclusion 16. RefeREnce .

5. Introduction The thyroid gland is largest gland of the body. The  thyroid gland , or simply the  thyroid , is an  endocrine gland in he neck , consisting of two  lobes connected by an  isthmus . It is found at the front of the neck, below the  adam's apple. The thyroid gland secretes three hormones , namely the two thyroid hormones ( thyroxine /T 4  and   Triiodothyronine /T 3 ) and  calcitonin . The thyroid hormones primarily influence the  metabolic rate a nd  protein synthesis , but they also have many other effects, including effects on development.  Calcitonin  plays a role in  calcium homeostasis .

6. History French chemist  B ernard Courtois  discovered iodine in 1811, and in 1896  E ugen Baumann  documented it as the central ingredient in the thyroid gland. He did this by boiling the thyroid glands of a thousand sheep, and named the precipitate, a combination of the thyroid hormones, 'iodothyrin'.  David marine  in 1907 proved that iodine is necessary for thyroid function. Thyroxine itself was first isolated in 1914 and synthesized in 1927, and tri- iodothyroxine in 1952. The conversion of T4 to T3 was discovered in 1970. 

7. The process of discovering TSH took place over the early to mid twentieth century. TRH was discovered by polish endocrinologist  A ndrew Schally  in 1970, contributing in part to his N obel prize in medicine in 1977.

8. Origin The thyroid gland is originated from endoderm of cephalic portion of the embryo’s alimentary canal. A sac like diverticulum first appears in the midline of the ventral surface of the pharynx. This glandular organ becomes bilobed but remains connected to the pharynx by thyroglossal duct. The thyroglossal duct become a solid stalk that usually atrophies. The two lateral lobes of human thyroid become solid masses of tissue and remains connected to each other by a narrow isthmus of tissue.

9. Location The gland is located immediately below the larynx and the anterior to upper parts or trachea.

10. Structure The thyroid gland is a butterfly-shaped organ that sits at the front of the neck. It is composed of two lobes, left and right, connected by a narrow isthmus. The thyroid weighs 25 grams in adults, with each lobe being about 5 cm long, 3 cm wide, and 2 cm thick and the isthmus about 1.25 cm in height and width. The gland is usually larger in women, and increases in size in pregnancy.

11. Microanatomy At the  microscopic level there are three primary features of the thyroid—follicles,  follicular cells and  parafollicular cells first discovered by G eoffery W ebsterson in 1664. Follicles :- Thyroid follicles are small spherical groupings of cells 0.02–0.9mm in diameter that play the main role in thyroid function. They consist of a rim that has a rich blood supply, nerve and lymphatic presence, that surrounds a core of  colloid  that consists mostly of thyroid hormone precursor proteins called  thyroglobulin , an  iodinated   glycoprotein .

12. Follicular cells :- The core of a follicle is surrounded by a single layer of follicular cells. When stimulated by thyroid stimulating hormone (TSH), these secrete the thyroid hormones T3 and T4. They do this by transporting and metabolising the thyroglobulin contained in the colloid. Follicular cells vary in shape from flat to cuboid to columnar, depending on how active they are.

13. Parafollicular cells :- Scattered among follicular cells and in spaces between the spherical follicles are another type of thyroid cell, parafollicular cells. These cells secrete  C alcitonin  and so are also called C cells.

14. Thyroid hormones The primary function of the thyroid gland is the production of iodine containing thyroid hormones. T3 and T4 and the peptide hormone calcitonin. T3 :- it contains three atoms of iodine per molecules secreted by follicular cells. T4 :- it contains four atoms of iodine per molecules and also secreted by follicular cells.

15. Difference between T3 and T4 :- ● thyroid secret about 80- 90 microgram of T4 but only 5 to 10 microgram of T 3 per day. ● T3 is 10x greater biological activity than t4 . Calcitonin :- produced by parafollicular cells. Antagonist to parathyroid hormone.

16. Biochemistry of thyroid hormone Iodine + tyrosine = thyroxine hormone Iodine trap by follicular cell. Synthesis of thyroglobulin a glycoprotein containing tyrosine residue for hormone synthesis. Binding of inorganic iodine to tyrosine residue in thyroglobulin. Synthesis of t3 and t4 from iodinated tyrosine. Storage of thyroglobulin containing the thyroid hormone in the lumen of the follicles.

17. Engulfing of the colloid bye follicular cells and hydrolysis of thyroglobulin to release thyroid hormone. Diffusion of t3 and t4 into the general circulation and their transport of target cell. Mechanism of action :-

18. Iodine and its uptake Iodine is a raw material essential for thyroid hormone synthesis. Ingested iodine is converted to iodide and absorbed. The basal membrane of thyroid gland has the specific ability to pump the iodine actively in the interior of the cell. Iodine trapping is stimulated by TSH.

19. Synthesis of thyroid hormone Thyroid hormone stores their hormones extracellularly. Thyroglobulin synthesized and change into follicle lumen. Iodide actively taken into cell and the released into lumen. Iodide oxidised to iodine. Iodine attach to tyrosine , mediated by peroxide enzyme. Iodinated tyrosine link together to form T3 and T4. Colloid is endocytosed and combined with lysosome. T3 and T4 are cleaved and diffuse into blood stream.

20. Transport and regulation T3 and T4 is transported by thyroxine binding globulin. Both T3 and T4 bind to target receptor but T3 is 10X more active than T4. Peripheral tissue convert T4 into T3 . Negative feedback regulation of t h release Raising TH level provide a negative feedback inhibition on release of TSH. Hypothalamic TRH also inhibited.

21. Mechanism of action The action of T3 is analogous to the model of steroid hormone action. Two type of thyroid receptor hormone TR α and TR β . TR β ge ne present on chromosome 3 and TR α on chromosome 17. TR β gene produced TR β 1 , TR β 2 proteins and TR α gene produced TR α 1, TR α 2 proteins. TR β 1 , TR β 2 and TR α 1 bind with T3 and regulate thyroid hormone response gene transcr i ption .

22. Action of mechanism work :- Calcium - ATPase activity – I ncrease . Thyroid hormone amino acid uptake – I ncrease . Deoxy glucose uptake – I ncrease . Na- K ATP ase stimulation – A ctivation .

23. Functions of Thyroid hormone Metabolic  :- the thyroid hormones increase the  basal metabolic rate (BMR)  and have effects on almost all body tissues. They increase the absorption in the gut,  generation ,  uptake b y cells , and  breakdown of glucose. They stimulate the  breakdown of fats , and increase the number of  free fatty acids . Cardiovascular :-   the hormones increase the rate and strength of the heartbeat. They increase the rate of breathing, intake and consumption of oxygen, and increase the activity of  mitochondria . Combined, these factors increase blood flow and the body's temperature.

24. Developmental :-   thyroid hormones are important for normal development. They increase the growth rate of young people . Thyroid hormones play a particularly crucial role in brain maturation during fetal development and first few years of postnatal life. ● The thyroid hormones also play a role in maintaining normal sexual function, sleep, and thought pattern s. Sexual function, including libido and the maintenance of a normal  M enstrual Cycle are influenced by thyroid hormones.

25. Disorders Hyp O rsecretion :- In adults :- Myxoedema or gull’s disease :- It is caused by deficiency of thyroid hormone in adults. This disease is characterized by puffy appearance due to accumulation of fat in the subcutaneous tissue because of low metabolic rate. This disease can be treated by administration of thyroid hormones.

26. IN CHILD (INFANTS) :- Cretinism :- This disorder is caused by deficiency of thyroid hormone in infant. A cretin has slow body growth and mental development of reduced metabolic rate.

27. Simple goitre :- Is caused by deficiency of iodine in diet because iodine is needed for the synthesis of thyroid hormone. It causes thyroid enlargement. It may lead to cretinism or myxedema. This disease is common in hilly areas . addition of iodine to the table salt prevents this disease.

28. hashimoto’s disease :- In this disease all the aspects of thyroid function are impaired . It is an autoimmune disease in which the thyroid gland is destroyed by autoimmunity.

29. HYPERSECRETION :- Graves’ disease :- It is a thyroid enlargement (goiter) in which the thyroid secretes excessive amount of thyroid hormone.It is characterized by exophthalmia (protrusion of eye balls because of fluid accumulation behind them).

30. Conclusion Thyroid is very essential hormone for organism it regulate many activities of the body like metabolic activity growth etc. It control many e biological functions. The deficiency of thyroid hormone can cause various disease.

31. References MA C E. HANDLEY & JON. E. LEVINE – Endocrinology (6th edi.) Introduction to Endocrinology – CHANDRA S. NEGI By Internet THANK YOU