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Thyroid gland

Thyroid gland. Structure : it is the largest endocrine gland in the body. It has butterfly shape. It consists of 2 lateral ( right & left ) lobes connected by a narrow isthmus.

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Thyroid gland

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  1. Thyroid gland • Structure : it is the largestendocrine gland in the body. It has butterfly shape. It consists of 2 lateral ( right & left ) lobes connected by a narrow isthmus. • Capsule :it is surrounded by : 1-a sheath derived from pretracheal layer of deep cervical fascia. this sheath attaches the gland to larynx and trachea, so it is responsible for movement of gland with lartnx up & down during deglutition.2-Fibrous capsule : is firmly attached to the gland. • Each lobe : is pear shaped, its apex is directed upward as far as the oblique line/ on lamina / of thyroid cartilage, while its base lies at the level of 5 or 6th tracheal ring.

  2. Thyroid gland • Isthmus : • It lies in the midline in front of2,3, and 4th tracheal rings. • A pyramidal lobe is often projects upwards from isthmus, usually to left of midline. It is connected to the hyoid bone by a fibromuscular band called levator glandulaethyroidae (remnant ofthyroglossal duct in embryo). This band may be absent.

  3. Relations of Lobes of Thyroid Gland : • Anterolaterally : sternothyroid, sternohyoid, superior belly of omohyoid, anterior border of sternocleidomastoid. • Postrolaterally : carotid sheath (common carotid artery + internal jugular vein + vagus nerve). • Medially :larynx + trachea, pharynx + esophagus, recurrent laryngeal nerve, cricothyroid muscle + its nerve supply, the external laryngeal nerve. • Posteriorly :the posteriorborder of eachlobe is related to superior & inferior parathyroid glands + anastomosis between superior & inferior thyroidarteries. Cross section of the neck at level of 6th cervical vertebra.

  4. Relations of Isthmus of Thyroid Gland : • Anteriorly : skin, fascia, anterior jugular veins, sternohyoids & sternothyroids. • Posteriorly : 2,3,and 4th tracheal rings. N.B :superior thyroid arteriesanastomose along its upperborder.

  5. Arteries of thyroid gland • Superior thyroid artery : a branch of external carotid artery, descends to upper pole of each lobe.-it is accompanied by superior thyroid vein + external laryngeal N. -it divides into anterior & posterior branches. The anterioranastomoses with its fellow of opposite side on the upper border of theisthmus, and the posterior one descends on posterior surface to anastomose with inferior thyroid artery. • Inferior thyroid artery : a branch of thyrocervical trunk from 1st part of subclavian A. -It turns medially and downward then, upward to posterior border of gland.-it is accompanied byrecurrent laryngeal nerve. • Thyroidea ima artery : if present, may arise from brachiocephalic artery or arch of aorta. It ascends in front of trachea to the isthmus. Anterior surface Posterior surface

  6. Veins and lymph drainage of thyroid gland : • Superior thyroid vein :drains into internal jugular vein. • Middle thyroid vein :drains into internal jugular vein. • Inferior thyroid vein :receives its tributaries from isthmus & lower poles of gland and descend in front of trachea to drain into left brachio-cephalic vein in the thorax. • The lymphfrom thyroid gland drains mainlyinto upper & lower deep cervical L.Ns. (along I.J.V)- Few into paratracheal L.Ns.(along side of trachea).

  7. Parathyroid glands • They are 2 pairs (2 superior + 2 inferior) of small endocrine glands embedded in the posterior aspect of the lobes of thyroid gland. • Superior parathyroid : lies at the middle of posterior surface of thyroid, they are more constant in position. • Inferior parathyroid : lies near base of thyroid gland. • They liewithin the fascial sheath, / embedded in the posterior wall of fibrous capsule. • Arterial supply : inferior thyroid vessels. • Lymph drainage : as thyroid gland.

  8. Cervical part of Trachea • It is a mobile cartilaginous & membranous tubestarting at the level of cricoid cartilage of larynx opposite 6th cervical vertebra. • It ends at the level of disc between4th & 5th thoracic vertebrae opposite the sternal angle.

  9. Relations : • Anteriorly : -isthmus of thyroid (in front of 2nd ,3rd ,and 4th rings) + inferior thyroid vein + thyroidea ima artery + jugular arch. -sterno thyroid + sternohyoid muscles + skin & fascia. • Posteriorly : recurrent laryngeal nerves + esophagus + vertebral column. • Laterally :lobes of thyroid gland (down as far as 5th or 6th ring ) + carotid sheath. • Blood supply : inferior thyroid vessels. • Lymph drainage : pretracheal + paratracheal L.Ns. • Nerve supply : vagus & recurrent laryngeal N. + sympathetic trunks.

  10. Cervical part of esophagus & its relations : • It is a muscular tube about 25 cm long extending from pharynx to stomach. It begins at the level of cricoid cartilage opposite 6th cervical vertebra, and endsto join the stomach in the abdomen. • Anteriorly :trachea,R.L.N. in groove between trachea & esophagus. • Posteriorly : prevertebral layer of deep cervical fascia, longus coli, and vertebral column. • Laterally :lobe of thyroid gland + carotid sheath. • Blood supply : inferior thyroid vessels. • Lymph drainage : deep cervical L.Ns. • Nerve supply : recurrent L.N. + sympathetic trunks.

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