ANATOMY OF THYROID GLAND presentation. Ppt

Surgical Anatomy of thyroid gland

1. ANATOMY OF THYROID PRESENTOR-DR SUCHAIT BHAT MODERATOR-DR TANISHA ARORA

2. Timeline of development of thyroid gland Gestational age Development Embryonic day 10 Endodermal thickening seen Embryonic day 16-17 Median and lateral anlagen visible Embryonic day 24 Median anlage has developed a thin, flask-like diverticulum, extending from the floor of the buccal cavity down to the fourth branchial arch. Embryonic day 30 Formation of bi-lobulated structure Embryonic day 40 Median and lateral anlagen have fused Degeneration of thyroglossal duct into fibrous stalk. Embryonic day 50 Lateral expansion Descent of the heart and thyroid reaches its final location, immediately anterior to the trachea. Precolloid phase starts

3. Gestational age Development Embryonic day 60 Definite shape Follicle formation Onset of thyroglobulin expression Gestational week 10 Colloid phase Thyroid hormone receptors detectable in brain Fibrous thyroglossal stalk disappears. Gestational week 11 Histogenesis of the thyroid is virtually complete. Foetal thyroid is capable of trapping and oxidizing iodide. Thyroglobulin present in foetal thyroid. Thyroxine can be detected in foetal serum.

4. Gestational age Development Gestational week 16 Increasing TSH secretion the principal regulator of thyroid hormone biosynthesis and secretion. Significant increase of thyroid hormone receptors in foetal brain. Gestational week 17.5 Structural maturity of thyroid gland Gestational week 20 Increasing organification of iodide and foetal T4 secretion Gestational week 24 T3 detectable Birth Gland weight 1.5 g

5. EMBRYOLOGY

6. POUCH 1- MIDDLE EAR CAVITY, EUSTACHIAN TUBE. POUCH 2- PALATINE TONSIL POUCH 3-INFERIOR PARATHYROID GLAND,THYMUS POUCH 4-SUPERIOR PARATHYROID GLANDS,ULTIMOBRANCHIAL BODY. POUCH 5- (OFTEN CONSIDERED RUDIMENTARY OR PART OF 4)- SOMETIMES CONTRIBUTES ULTIMOBRANCHIAL BODY.

7. External branch of the superior laryngeal nerve(EBSLN) lies deep to the upper pole of the gland as it passes superficial to the cricothyroid muscle. Lateral:upper pole of the gland and the superior thyroid vessels Medial : midline. Floor : cricothyroid muscle Roof :strap muscles, it usually contains the external laryngeal nerve running on cricothyroid .

8. Thyroid gland is the earliest endocrine organ to appear in a mammalian development. It is formed from both the endodermal cells of the primitive pharynx , known as the median anlage, and from neural crest cells from the lateral anlage or ultimobranchial bodies. The median anlage develops from a midline thickening of the ventral surface of the endodermal epithelium of the primitive pharynx between the 1 st and 2 nd branchial arches adjacent to the developing myocardium. A diverticulum forms from this thickening at about the 16 th or 17 th gestational day. This out- pouching expands laterally at its distal tip to form a rudimentary bilobed structure. It is pulled into position by the descent of the heart and reaches its final location, immediately anterior to the trachea in the 7 th gestational week.

9. Initially the descending thyroid remains attached to the pharyngeal floor by a tubular stalk. During the 7 th to 10 th week the lumen of the stalk becomes filled with cords of thyroid progenitor cells but eventually fragments and disappears before birth. The residum of the attachment of the stalk forms the foramen caecum in the midline at the junction of the posterior 3 rd and anterior 2/3 rd of the tongue. The two lateral anlage , also known as the ultimobranchial bodies , are thought to develop as evaginations of the 4 th pharyngeal pouches. They become separated from them by attenuation and rupture from the common pharyngobranchial duct . The ultimobranchial bodies fuse with the median thyroid anlage by the 6 th week of development, contributing approximately 10% of the mass of the thyroid gland. The ultimobranchial bodies give rise to the calcitonin-producing parafollicular cells (C cells). The exact origin of these cells in humans remains controversial although they are thought to migrate from the neural crest.

10. HISTOLOGY The thyroid gland is enclosed by the thyroid capsule, which is a thin, dense layer of connective tissue that sends septa into the thyroid parenchyma, subdividing the thyroid gland into several lobules. Each thyroid lobule contains 20 to 40 round to oval follicles, measuring 30 to 500 microns in diameter. Each thyroid follicle is lined by cuboidal epithelial cells and contains a central store of colloid secreted by the epithelial cells under the influence of the thyroid stimulating hormone (TSH). On prolonged and excessive thyroid-stimulating hormone (TSH) stimulation the follicular cells become hypertrophied and hyperplastic and they adopt a more columnar shape. When the gland is relatively inactive, the cells are flattened and the colloid is abundant, dense and homogenous in appearance.

11. Precolloid stage :(7-13 weeks) starts from the first appearance of immature follicular cells to the beginning of differentiation into cells that are capable of producing colloid.In simple terms,cells are still immature,starting to specialize and getting ready to produce colloid. Colloid formation stage (13-14 weeks):In this stage,cells actively start secreting material into the follicle lumen,forming the first colloid. Follicular maturation stage (after 14 weeks):thyroid follicles become fully functional- making,storing,and preparing to release thyroid hormones.

12. Anatomy of thyroid Thyroid gland : made up of two lateral lobes, which extend from sides of the thyroid cartilage down to 6 th tracheal ring. In addition there is often a pyramidal lobe which projects up from isthmus,usually on the left hand side. situated : lower anterior neck straddling the upper trachea. It is the largest endocrine organ in the body. weight:15–20 g in adulthood. Weighs larger in females than males and further increases in size during mensuration and pregnancy . Highly vascular, reddish-brown, bi-lobed structure with each lobe joined together by a narrow isthmus . Dimensions : Each lobe is a pear-shaped, measuring approximately 5 cm in length, 3 cm in width and 1.5 cm in depth. The apex of each lobe is narrow and extends beneath the sternothyroid muscle up to its insertion on the oblique line of the thyroid cartilage . The more rounded lower pole,the more it extends down to the level of the 4 th or 5 th tracheal ring. Medial:trachea and oesophagus , lateral:carotid sheath. The isthmus overlies the 2 nd to 4 th tracheal rings.

13. The thyroid gland , together with the oesophagus and trachea , is present in a visceral layer of deep fascia known as the pretracheal fascia . It is attached superiorly to the hyoid bone and extends inferiorly into the mediastinum , fusing with the fascia surrounding the aorta , pericardium and parietal pleura at the level of the carina . Laterally the fascia blends with the carotid sheath . Anteriorly, the fascia forms a distinct layer separating the thyroid from the strap muscles. Posteriorly, it merges with the prevertebral fascia . On the posterior aspect of the isthmus the fascia is sometimes known as the anterior tracheal ligament and is perforated by small tracheal vessel.

14. Lateral ligament of the thyroid/posterior Suspensory ligament of Berry : It is a condensation of fascia connecting the cricoid and the 1st and sometimes the 2 nd tracheal ring to the posteromedial aspect of each thyroid lobe.This pair of strong condensed connective tissue binds the gland firmly to each side of cricoid cartilage and upper tracheal rings. Pretracheal fascia , which is part of deep cervical fascia splits to invest the gland. These structures (ligament of Berry and pretracheal fascia) are responsible for thyroid gland moving with deglutition.

15. Capsules of thyroid gland True capsule :surrounds the gland. False capsule: loose areolar tissue derived from the middle layer of deep cervical fascia which ensheaths larynx ,trachea and thyroid.

16. Point to be noted If a pyramidal lobe is present on the side of surgery, it should be removed with the main lobe to avoid leaving behind functioning thyroid tissue.

17. Tubercle of zuckerkandl and its surgical importance The tubercle of Zuckerkandl is a lateral or posterior projection of the thyroid lobe which can be identified in up to 60% of surgical dissections.It is thought to represent the point of embryological fusion of the ultimobranchial body and median anlage. Surgical importance:The recurrent laryngeal nerve most often runs medial to it. The superior parathyroid gland is usually attached to its cranial aspect. When enlarged as part of the goitre , a significant portion of thyroid tissue may be inadvertently left behind if subtotal thyroidectomy is performed.

18. HOW TO IDENTIFY RLN DURING SURGERY LACK OF COLOUR-WHITE IN COLOUR LACK OF ELASTICITY-DOESN’T STRETCH LIKE ARTERIES LACK OF PULSATION-UNLIKE ARTERIES,IT DOESN’T PULSATE LONGITUDNAL COURSE-RUNS UPWARD IN A STRAIGHT COURSE RIDDLE S TRIANGLE/BEAHRS TRIANGLE :SURGICAL LANDMARK TO FIND RLN SUPERIORLY :INFERIOR THYROID ARTERY LATERALLY: CAROTID ARTERY MEDIALLY: TRACHEA

19. STRAP MUSCLES AND THEIR NERVE SUPPLY SUPERFACIAL LAYER: 1. STERNOHYOID : ORIGIN- MANUBRIUM OF STERNUM AND MEDIAL PART OF CLAVICLE. INSERTION- BODY OF HYOID BONE ACTION- DEPRESSES HYOID AFTER SWALLOWING 2. OMOHYOID: ORIGIN-SUPERIOR BORDER OF SCPULA INSERTION- BODY OF HYOID BONE ACTION- DEPRESSES AND RETRACTS HYOID DEEP LAYER: 3.STERNOTHYROID: ORIGIN- POSTERIOR SURFACE OF MANUBRIUM OF STERNUM INSERTION: THYROID CARTILAGE OF LARYNX ACTION: DEPRESSES LARYNX 4.THYROHYOID : ORIGIN- THYROID CARTILAGE INSERTION- HYOID BONE ACTION- DEPRESSES OR ELEVATES LARYNX WHEN HYOID IS FIXED

20. All strap muscles are supplied by ansa cervicalis(c1-c3) except thyrohyoid which is supplied by branch of c1 via hypoglossal nerve.

21. Arterial supply Superior thyroid artery is a branch of the external carotid artery , it enters the upper pole of the gland, divides into anterior and posterior branches and anastomoses with ascending branch of inferior thyroid artery . Since the upper pole is narrow, ligation is easy. Inferior thyroid artery is a branch of thyrocervical trunk (branch of subclavian artery)but in 15% it arises directly from subclavian artery and enters the posterior aspect of the gland. It supplies the gland by dividing into 4 to 5 branches which enter the gland at various levels (not truly lower pole). Inferior thyroid artery used to be ligated well away from the gland to avoid damage to RLN. However, ligation of these arteries on both sides will cause permanent hypoparathyroidism. Hence, the current practice is to identify and ligate the branches of inferior thyroid artery (3-4) separately. Thyroidea ima artery is a branch of either brachiocephalic trunk or direct branch of arch of aorta and enters the lower part of the isthmus in about 2 to 3% of the cases.

22. Venous drainage Superior thyroid vein drains the upper pole and enters the internal jugular vein. The vein follows the artery. Middle thyroid vein is a single, short and wide and drain into internal jugular vein. Inferior thyroid veins are multiple and form a plexus which drain into right and left brachiocephalic vein. They do not accompany the artery. kocher's vein is rarely found (vein in between middle and inferior thyroid vein).

23. The RLN can be found deep to the inferior thyroid artery (40% of the cases), superficial to the RLN (20% of the cases), or between the branches of the inferior thyroid artery (35% of the cases).

24. Nerves in relationship with thyroid gland Superior laryngeal nerve : The vagus nerve gives rise to superior laryngeal nerve, which separates from it at skull base and divides into two branches. The larger internal laryngeal nerve is sensory to the supraglottic larynx. The smaller external laryngeal nerve runs close to the superior thyroid vessels and supplies cricothyroid. This nerve is away from the vessels near the upper pole. Hence, during thyroidectomy, the upper pedicle should be ligated as close to the thyroid as possible. Recurrent laryngeal nerve (RLN) is a branch of vagus , hooks around ligamentum arteriosum on the left and subclavian artery on the right and runs in the tracheoesophageal groove near the posteromedial surface. Close to the gland, the nerve lies in between (anterior or posterior) branches of inferior thyroid artery. Recurrent laryngeal nerve (RLN) is close to the trachea on the left than the right.

25. Lymphatic drainage The upper poles of the gland together with the isthmus and the pyramidal lobe drain superiorly, terminating in the lateral neck in levels II/III. Subcapsular lymphatic plexus drains into pretracheal nodes and prelaryngeal nodes which ultimately drain into lower deep cervical nodes and mediastinal nodes . The chief lymph nodes are middle and lower deep cervical lymph nodes (Levels III and IV). lateral aspect of each lobe drains into levels III and IV. Supraclavicular nodes and nodes in the posterior triangle can also be involved in malignancies of the thyroid gland, especially papillary carcinoma of thyroid.