Anatomy & physiology of thyroid gland

Thyroid gland - Anatomy and physiology

1. ANATOMY AND PHYSIOLOGY OF THYROID GLAND BY:- Dr. JINU IYPE 2 nd YEAR POSTGRADUATE DEPARTMENT OF ENT

2. Scott Brown 7th edition vol 1 Textbook otorhinolaryngology- Zakir Hussain Textbook of Sabiston 19 edition Guyton textbook of physiology 12 th edition

3. FROM GREEK thyreoeides = SHIELD SHAPE

4. Butterfly/H shaped Brownish-Red Highly vascular Ductless gland Adult gland weighing 20 to 25gm. Larger in female Enlarges further during puberty, menstruation & pregnancy

5. Consist of right and left cone shaped lobes 5*3*2cm Connected by a narrow region of gland - Isthmus 1.2*1.2cm Situated anteriorly in the visceral compartment of the neck at the level of C5-T1 vertebrae

6. Location: C 5 T 1 / 4 th or 5 th tracheal ring 2 nd Tracheal rings 4 th

7. DEVELOPMENT OF THYROID GLAND Median endodermal thyroid diverticulum ( median anlage ) Foramen caecum Thyroglossal duct ( usually reabsorbed after 6 weeks of age ) The very distal end of this remnant may be retained and mature as a pyramidal lobe in the adult thyroid

8. THE THYROID begins to function- end of 3month, at which time, the first follicles containing colloid can be seen Ultimobranchial body gives parafollicular C cells to thyroid gland - Parafollicular cells (C cells) from the neural crest reach the thyroid via the ultimobranchial body- arises from 4 th & 5 th Brachial pouch . Superior parathyroid gland- arises from 4 th Brachial pouch.

9. Inferior parathyroid gland – arises from 3 rd brachial pouch

10. A lateral or posterior projection of the thyroid lobe, known as the tubercle of Zuckerkandl , identified in up to 60 % of surgical dissections It is represent the point of embryological fusion of the ultimobranchial body and median anlage . Its surgical importance is : ( a) RLN runs medial to it; ( b) the superior parathyroid gland attached to its cranial aspect;

11. CAP S ULES True capsule (fibrous) – contains the parenchyma & sends fine s eptae between lobules of the thyroid gland. Arteries and plexus of veins deep to it False capsule – pretracheal fascia

12. Deep cervical fascia

13. Suspensory ligament of berry The pretracheal layer is thin along the posterior border of the lobes, but thick on the inner surface of the gland where it forms a suspensory ligament of berry which connects the gland to the cricoid cartilag e

14. Why thyroid moves with deglutition? During 1st stage of deglutition Hyoid bone moves up Pulls pre-tracheal fascia up This pulls ligament of berry upward This pulls thyroid upward

15. A fibromuscular band levator glandulae thyroideae descend from the body of the hyoid bone to isthmus or to pyramidal lobe

16. RELATIONS The lobes are conical in shape having Apex Base Three surfaces : Lateral, Medial, Posterolateral Two borders : Anterior & posterior

17. APEX : Directed upward & slightly laterally. limited superiorly – sternothyroid on the oblique line of thyroid cartilage BASE : At the level of 4 th or 5 th tracheal length

18. Lateral surface – convex, Covered with sternohyoid, SCM, superior belly of omohyoid , s ternothyroid

19. Medial surface- T rachea & O esophagus 2 muscles - cricothyroid, inferior c onstrictor 2 nerves - external laryngeal & recurrent laryngeal N P o s t e r o l a t e r a l surface – carotid sheath

20. Ant border – anterior branch of S uperior t hyroid a rtery

21. P osterior border - Thick and round I nf e r ior T hyroid A rtery A nastomosis b/w STA & ITA P arathyroid glands T horacic duct on left

22. ISTHMUS 2 surfaces A nt & P ost 2 borders Sup & Inf A nterior surface – skin & fascia - anterior jugular veins - R & L sternohyoid & sternothyroid P o s t e r i o r surface- 2 nd – 3 rd tracheal rings Sup border – anastomosis b/w R & L S up T hyroid A rtery Inf border - ITV leave

23. Blood supply Superior thyroid artery : 1 st anterior branch of external carotid artery Runs downwards & forward with close relation with external laryngeal nerve Pierces pretracheal fascia- upper pole of lobe anterior Posterior branch

24. Anterior branch descends on the anterior border of the lobe anastomosing branch which runs along the upper border of the isthmus to anastomosis with opp. side

25. Posterior branch – P osterior border of the lobe Anastomosis with ascending branch of inferior thyroid artery Superior thyroid artery Supplies Upper 1/3 rd of the lobe U pper ½ of the isthmus

26. I nferior thyroid artery : Branch of thyrocervival trunk Subclavian artery Runs forward then medially & finally downward to reach lower pole of the gland. Pa ss behind carotid sheath, Middle Cervical Ganglion and in front of vertebral vessels Close related Recurrent laryngeal Nerve

27. Artery divides into 4 or 5 Glandular branch pierces the fascia separately to reach the lower part of the gland Ascending branch anastomoses with posterior branch of superior thyroid artery Supply Parathyroid gland Inferior thyroid artery Supplies Lower 2/3 rd of the lobe Lower ½ of the isthmus & Parathyroid gland

28. THYROIDEA IMA ARTERY ( lowest thyroid artery ) 12 % present ascending in front of the trachea to end at the isthmus. most commonly arises from the brachiocephalic artery it can also originate from the aorta, the right common carotid , subclavian or internal thoracic arteries .

29. VENOUS DRAINAGE : The superior, middle and inferior thyroid vein Superior thyroid vein (STV) Accompany Sup Thyroid Artery Drain to IJV/ facial vein

30. Middle thyroid vein Very short , may be double or absent . They receive blood from the inferior and antero -lateral part of the gland as well as the larynx and trachea . most commonly cross the common carotid artery Drain to IJV

31. Inferior thyroid vein Plexus anterior surface of the trachea on leaving the gland. usually drain into right and left inferior veins, superior vena cava or left brachiocephalic

32. 4 TH VEIN OF KOCHER’S Seen b/w middle and inferior thyroid vein drain into IJV

33. LYMPHATIC DRAINAGE The thyroid gland contains a rich network of lymphatics The lateral aspects of the gland drain into levels III and IV and those of the posterior triangle (level V). The more medial aspects of the gland also drain into the nodes of the anterior compartment of the neck (level VI), drain into those of the superior mediastinum (level VII).

34. Nerve supply: Parasympathetic fibers –from V agus Sympatheticfibers – from superior, middle, and inferior ganglia of the sympathetic trunk Enter the gland along with the blood vessels.

35. Recurrent laryngeal nerves The recurrent laryngeal nerve is variable in size 1.5-4 mm in diameter . Identified by its whitish appearance, characteristic longitudinal vessel flattened , rounded surface . In up to 39 % of cases the nerve divides into 2 (and occasionally up to 6) terminal branches between 6 and 35 mm from the cricoid cartilage.

36. A non recurrent laryngeal nerve is found in 0.2-0.4% of patients It tends to be thicker than a normally sited nerve Usually associated with a vascular anomaly of the subclavian artery on the right side Transposition of the great vessels on the left side.

37. Recurrent laryngeal nerves Branches of the vagus nerve, which supply all the intrinsic muscles of the larynx except the cricothyroid muscle .(External laryngeal N – Superior Laryngeal N) They also supply sensory fibres to the mucous membrane below the level of the vocal folds Accidental damage to this nerve during surgery causes ipsilateral vocal cord paralysis & difficulty in phonation

38. Left Recurrent laryngeal nerves The approximate length of the left RLN is 12 cms The nerve leaves the vagus in the mediastinum anterior to the arch of the aorta passing behind the ligamentum arteriosum & then posteriorly under the concavity of the arch before passing superiorly to lie in the tracheo -oesophageal groove .

39. It most usually passes behind the inferior thyroid artery Then posterior to the ligament of Berry before passing under or between the fibres of the cricopharyngeal part of the inferior constrictor

40. Right Recurrent laryngeal nerves The approximate length of right RLN 6 cms Rt side it originates from vagus crosses first part of subclavian artery . More oblique course to the tracheo - oesophageal groove .

41. SUPERIOR LARYNGEAL NERVE Arises from inferior ganglion of vagus Descends behind internal carotid artery At the level of greater cornua of hyoid it divides:- I nternal branch(sensory) E xternal branch(motor)

42. The external branch of the superior laryngeal nerve, which supplies the cricothyroid muscle , runs parallel to the superior thyroid vessels The Internal branch of the superior laryngeal nerve supply sensory fibres to the mucous membrane above the level of the vocal folds

43. External branch of superior laryngeal nerve and joll’s triangle Joll's triangle is used to identify the location of external branch of superior laryngeal nerve during thyroid surgeries. Damage to this nerve during the surgical procedure may reduce the voice range in those patients. This triangle is also known as sternothyrolaryngeal triangle.

44. Boundaries of Joll's triangle : Lateral - Upper pole of thyroid gland and superior thyroid vessels Superior - Attachment of the strap muscles and deep investing layer of fascia to the hyoid Medial - Midline Floor - Cricothyroid muscle External branch of superior laryngeal nerve lies within this triangle.

45. Beahrs Triangle or Riddle’s triangle Boundaries Medial :The RL nerve in the lower part of tracheo - oesophageal groove Lateral :Common carotid Superior: Inferior thyroid artery

46. Microscopic anatomy The thyroid gland consists mainly of follicular cells , one cell thick around a central pool of colloid to form follicles . The follicles spherical in shape & 0.02- 0.9 mm in diameter A thyroid lobule consists of 20 to 40 follicles and is supplied by a lobular artery.

47. When the gland is relatively inactive, the cells are flattened and the colloid is abundant, dense. On prolonged and excessive TSH stimulation, the follicular cells become hypertrophied and hyperplastic and they adopt a more columnar shape. This cellular enlargement is associated with development of microvilli which helps in reduction in the size of the follicular lumen.

48. PHYSIOLOGY OF THE THYROID The thyroid follicles secretes tri- iodothyronine ( T3 ) and thyroxin(T4 ) Synthesis involves combination of iodine with tyrosine group to form mono and di- iodotyrosine which are coupled to form T3 andT4 . The hormones are stored in follicles bound to thyrogobulin

49. When hormones released in the blood they are bound to plasma proteins and small amount remain free in the plasma The metabolic effect of thyroid hormones are due to free (unbound)T3 and T4 . 90%of secreted hormones is T4 but T3is the active hormone so , T4is converted to T3 peripherally.

50. Regulation of thyroid gland metabolism Circulating T 3 and T4 exert -- ve feedback mechanism on hypothalamus and anterior pituitary gland So , in hyperthyroidism where hormone level in blood is high ,TSH production is suppressed and vice versa.

51. STEPS OF THYROID HORMONE SYNTHESIS Thyroglobulin Synthesis Iodine trapping Oxidation Iodination Coupling Storage & Release

52. 1. Thyroglobulin Synthesis Endoplasmic reticulum and Golgi apparatus in the follicular cells of thyroid gland synthesize and secrete thyroglobulin continuously. Thyroglobulin molecule is a large glycoprotein containing 140 molecules of amino acid tyrosine . After synthesis, thyroglobulin is stored in the follicle .

53. 2. Iodine trapping Iodide is actively transported from blood into follicular cell , against electrochemical gradient. This process is called iodide trapping . Iodide is transported into the follicular cell along with sodium by Sodium iodide (Na + / I - ) symporter , which is also called iodide pump. Whereby two sodium ions are transported for each iodide ion.

54. The accumulated iodide in the follicular cells is then transferred to the apical plasma membrane down an electrochemical gradient. TSH stimulation increases adenosine triphosphate (ATP) and ATPase activity at the apex of the cell increasing the efflux of iodide into the colloid down a further electrical gradient . Iodine available through certain foods ( eg , seafood, bread, dairy products), iodized salt, or dietary supplements etc

55. 3. Oxidation of Iodide Iodide must be oxidized to elementary iodine, because only iodine is capable of combining with tyrosine to form thyroid hormones. The oxidation of iodide into iodine occurs inside the follicular cells in the presence of thyroid peroxidase .

56. 4. Iodination of Tyrosine Iodine is transported from follicular cells into the follicular cavity, where it binds with thyroglobulin. Then , iodine (I) combines with tyrosine, which is already present in thyroglobulin Tyrosine is iodized first into monoiodotyrosine (MIT) and later into di- iodotyrosine (DIT )- thyroglobulin ( Tg ) tyrosine residues

57. 5. Coupling Reactions i . One molecule of DIT and one molecule of MIT combine to form tri- iodothyronine (T3 ) DIT + MIT = Tri- iodothyronine (T3) ii. O ne molecule of MIT and one molecule of DIT combine to produce another form of T3 called reverse T3 or rT3. Reverse T3 is only 1% of thyroid output MIT + DIT = Reverse T3 iii . Two molecules of DIT combine to form tetraiodothyronine (T4) thyroxine . DIT + DIT = Tetraiodothyronine or Thyroxine (T4)

58. FUNCTIONS OF THYROID HORMONES Action on basal metabolic rate (BMR) Thyroid hormones (specifically T 3 ) regulate rate of overall body metabolism T 3 increases basal metabolic rate Calorigenic effects T 3 increases oxygen consumption by most peripheral tissues Increases body heat production

59. 2. Action on carbohydrate metabolism Thyroxine stimulates almost all processes involved in the metabolism of carbohydrate. Thyroxine : i. Increases the absorption of glucose from GI tract ii. Enhances the glucose uptake by the cells, by accelerating the transport of glucose through the cell membrane iii. Increases the breakdown of glycogen into glucose iv. Accelerates gluconeogenesis.

60. 3. Action on fat metabolism Thyroxine decreases the fat storage by mobilizing it from adipose tissues and fat depots. Thus, thyroxine increases the free fatty acid level in blood.

61. 4. Action on Growth and Development Thyroid hormone i s essential for normal brain development Essential for childhood growth Untreated congenital hypothyroidism or chronic hypothyroidism during childhood can result in incomplete development and mental retardation

62. 5. Action on CNS Thyroid hormones are essential for neural development and maturation and function of the CNS Decreased thyroid hormone concentrations may lead to alterations in cognitive function Patients with hypothyroidism may develop impairment of attention, slowed motor function, and poor memory Thyroid-replacement therapy may improve cognitive function when hypothyroidism is present

63. 6.Action on Bone Growth – T 3 also may participate in osteoblast differentiation and proliferation, and chondrocyte maturation leading to bone ossification

64. 7.Action on f emale Reproductive System Normal thyroid hormone function is important for reproductive function – Hypothyroidism may be associated with menstrual disorders, infertility, risk of miscarriage , and other complications of pregnancy

65. 8.ACTION ON GASTROINTESTINAL TRACT Generally , thyroxine increases the appetite and food intake. It also increases the secretions and movements of GI tract. So , hypersecretion of thyroxine causes diarrhoea and the lack of thyroxine causes constipation.

66. 9 . Actions on:- Heart In Increase cardiac output, heart rate Adipose tissue Stimulate lipolysis Muscle Increase protein breakdown

67. APPLIED ANATOMY Presence of thyroidae ima A- chance of profuse bleeding procedures in neck below isthmus Thyroglossal cysts – R emnants of thyroglossal ducts at any point in the way of descent ,(midline near hyoid ) Pyramidal lobe and presence of levator glandulae thyroidae

68. Ectopic thyroid glands – lingual/higher placed Non neoplastic, noninflammatory enlargement – goiter pressure symptoms and nerve involvments are common in goiter and carcinoma

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