thyroid anatomy copy.pptx

thyroid gland anatomy and physiology best and easy explained..

1. Anatomy of the thyroid gland

2. Topic outline Definition: Description:Location,Structure,Lobes, Relationships with neighbouring structures,vascular anatomy,lymphatics of thyroid n parathyroid glands Clinical notes:Hypo n hyper thyroidism,hyperparathyroidism

3. Background The thyroid gland weighs 10 to 20 grams in normal adults.  Thyroid volume measured by ultrasonography (US) is slightly greater in men than women it increases with age and body weight. it decreases with increasing iodine intake. The thyroid is one of the most vascular organs in the body.

4. … The normal thyroid gland is immediately caudal to the larynx and encircles the anterolateral portion of the trachea. The thyroid is bordered by the trachea and esophagus medially and the carotid sheath laterally. The sternocleidomastoid muscle and the three strap muscles (sternohyoid, sternothyroid, and the superior belly of the omohyoid) border the thyroid gland anteriorly and laterally

5. … It consists of two lobes connected by a narrow isthmus.  Each lobe is pyramidal in shape, with its apex directed upward and its base directed downward.  The isthmus is the narrow part of the gland connecting the two lobes.  A small pyramidal lobe projecting upward from the isthmus is often present to the left of the midline.

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7. Transverse view: relationship to other NB structures in neck

8. Histology Under middle layer of deep cervical fascia (pretracheal)  thyroid inner true capsule  thin and closely adherent to the gland capsule extensions within the gland form septae, dividing it into lobes and lobules lobules are composed of follicles = structural units of the gland  layer epithelium enclosing a colloid-filled cavity colloid (pink on H&E stain) contains an iodinated glycoprotein, iodothyroglobulin (precursor of thyroid hormones).

9. Follicles = variable size surrounded by dense plexuses of fenestrated capillaries, lymphatic vessels, and sympathetic nerves.

10. … Epithelial cells = 2 types: principal (ie: follicular) – formation of colloid (iodothyroglobulin) parafollicular (ie: C cells -clear, light), lie adjacent to follicles w/in basal lamina  produce calcitonin

11. Arterial supply The arterial blood supply to the thyroid gland is primarily from the right and left superior and inferior thyroid arteries, derived from the external carotid arteries and thyrocervical trunk, respectively. The venous drainage consists of the superior, middle, and inferior thyroid veins that drain into the internal jugular vein and innominate vein Superior thyroid artery is the first branch off the external carotid artery. It extends inferiorly to the superior pole of the thyroid lobe. In addition to supplying the thyroid, the superior thyroid artery is the primary blood supply to approximately 15 percent of superior parathyroid glands. The superior thyroid artery is a landmark for identification of the superior laryngeal nerve, which courses with the artery until approximately 1 cm from the superior thyroid pole

12. … Inferior thyroid artery is a branch of the thyrocervical trunk which arises from the subclavian artery. The inferior thyroid artery courses posterior to the carotid artery to enter the lateral thyroid. The point of entry can extend from superior to inferior thyroid poles. The inferior thyroid artery also supplies the inferior parathyroid glands and approximately 85 percent of superior parathyroid glands.. Thyroidea ima artery is found in approximately 3 percent of individuals and arises from the aortic arch or innominate artery and courses to the inferior portion of the isthmus or inferior thyroid poles. Surgical control of the thyroidea ima artery is essential during thyroidectomy. The thyroidea ima artery can be quite enlarged in patients with thyroid disease such as goiter or hyperthyroidism

13. Venous drainage Venous drainage Superior thyroid vein: It arises from the upper part of the lobe. It ends into the internal jugular vein. Middle thyroid vein: It arises from the middle of the lobe. It ends into the internal jugular vein. Inferior thyroid veins: Arise from the isthmus and lower parts of the lobes. Descend in front of the trachea. End into the left brachiocephalic vein.

14. Lymphatic drainage The lymphatic vessels of the thyroid gland drain into: 1) Pretracheal lymph nodes. 2) Paratracheal lymph nodes. - The efferent of these nodes drain into the deep cervical lymph nodes.

15. Recurrent laryngeal nerve Recall: innervates all larynx except cricothyroid Closely assoc with ITA (see next slides for details) NB: ‘ non recurrent LN ’ ~5/1000 pt’s on R side When retroesophageal R SCA (right subclavian artery) from dorsal aortic arch NRLN - branches from X at ~ cricoid cartilage directly enters the larynx without looping around SC L sided - only when R aortic arch and ligamentum arteriosum concurrent w/ L retroesophageal subclavian artery .

16. … The left and right  recurrent laryngeal nerves  lie in close proximity to the thyroid gland and care must be taken not to damage them during thyroid surgery. They branch from their respective  vagus nerve  within the chest and hook around the right subclavian artery (right RL nerve), or the arch of aorta (left RL nerve). The recurrent laryngeal nerve then travels back up the neck, running between the trachea and oesophagus in the  tracheoesophageal groove . It then passes underneath the  thyroid gland  to innervate the larynx.

17. … The RLN may course anterior or posterior to the inferior thyroid artery. In some cases, the RLN may branch into both an anterior and posterior position Vocal cord paresis or paralysis due to iatrogenic injury  of the recurrent laryngeal nerve (RLNI) is one of the main problems in thyroid surgery. Although many procedures have been introduced to prevent the nerve injury, still the incidence of recurrent laryngeal nerve palsy varies between 1.5-14%. Injury to the recurrent laryngeal nerve has the potential to cause  unilateral vocal cord paralysis . Patients with this typically complain of new-onset hoarseness, changes in vocal pitch, or noisy breathing.

18. . Thyroid Pathology Hyperthyroidism, commonest is Grave’s disease (autoimmune) Hypothyroidism In childhood leads to cretinism Endemic goiter from insufficient iodine in diet Adult hypothyroidism (myxedema): autoimmune

19. 25 Exophthalmos of Grave’s disease Enlarged thyroid (goiter) from iodine deficiency

20. Parathyroid gland Normal parathyroid glands are approximately the size of a grain of rice or a lentil. Normal glands are usually about 5 by 4 by 2 millimeters in size and weigh 35 to 50 milligrams. Enlarged parathyroid glands can be 50 milligrams to 20 grams in weight, most typically weighing about 1 gram and 1 centimeter in size Normal superior parathyroid glands are usually located on the posterior-lateral surface of the middle to superior thyroid lobe. They lie under the thyroid superficial fascia, posterior to the recurrent laryngeal nerve and can be visualized by carefully dissecting the thyroid capsule in this region.

21. … The two inferior parathyroid glands reside in the anterior mediastinal compartment, anterior to the recurrent laryngeal nerve. They are most often found in the thyrothymic tract, or just inside the thyroid capsule on the inferior portion of the thyroid lobes The superior parathyroid glands receive most of their blood supply from the inferior thyroid artery and also are supplied by branches of the superior thyroid artery in 15 to 20 percent of patients. The inferior parathyroid glands receive their end-arterial blood supply from the inferior thyroid artery.

22. PTH function

23. Hyperparathyroidism Primary Hyperparathyroidism Normal feedback of Ca disturbed, causing increased production of PTH Secondary Hyperparathyroidism Defect in mineral homeostasis leading to a compensatory increase in parathyroid gland function Tertiary Hyperparathyroidism After prolonged compensatory stimulation, hyperplastic gland develops autonomous function

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