ANATOMY OF THYROID GLAND:
- Thyroid gland is an ENDOCRINE gland.
- It consists of two lobes (right and left) which are connected together by ISTHMUS. Occasionally there may be a third lobe called PYRAMIDAL LOBE. This is an upward projection which may either arise from isthmus or from any one of the lobe.
- Sometimes a fibromuscular band may extend from hyoid bone to the isthmus or to the pyramidal lobe. This is called LEVATOR GLANDULAE THYROIDEA.
- Rarely accessory thyroid gland may be present.
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SITUATION:
- It is situated in the anterior of the neck.
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EXTENT:
- The gland extends from fifth cervical vertebra to the first thoracic vertebra.
- Each lobe extends from middle of the thyroid cartilage to the fourth or fifth tracheal ring.
- Isthmus extends from second to fourth tracheal ring.
DIMENSIONS:
- Each lobe measures about 5*2.5*2.5.
- Isthmus measures about 1.2*1.2.
WEIGHT:
- The gland usually weighs about 25 grams.
- It is larger in case of females because there is increase in the size of the gland during menstruation and pregnancy.
COMPARISON OF CAPSULE OF THYROID GLAND AND PROSTATE GLAND:
- Both of them has a true and a false capsule.
- True capsule is the peripheral condensation of the connective tissue.
- False capsule is formed by the Pretracheal Fascia. It is thick on the inner surface and it is called SUSPENSORY LIGAMENT OF BERRY. This connects the gland to the cricoid cartilage. This is reason for the movement of thyroid cartilage during deglutition.
- The major distinguishing feature is the venous plexus. In thyroid gland it lies DEEP TO THE TRUE CAPSULE. In prostate gland it lies IN BETWEEN THE TWO CAPSULES.
RELATIONS:
- Each lobe has
- An apex
- Base
- Three surfaces namely lateral, medial and posterolateral
- Two borders namely anterior and posterior.
- Apex is directed upwards and its attachment is restricted by the attachment of sternothyroid to the oblique line of thyroid cartilage.
- Base is related to the 4th or 5th tracheal ring.
- Lateral surface is related to the muscles like sternohyoid, sternothyroid, anterior border of sternocleidomastoid, superior border of omohyoid.
- Medial surface is related to TRACHEA, OESOPHAGUS, inferior constrictor and cricothyroid, external laryngeal and recurrent laryngeal nerve.
- Posterior surface is related to the carotid sheath.
- Anterior border is related to the anterior branch of superior thyroid artery.
- Posterior border is related the inferior thyroid artery, parathyroid gland, thoracic duct.
Relations of the isthmus:
- Isthmus has 2 surfaces (anterior and posterior) and 2 borders(superior and inferior)
- Anterior surface is covered by the right and left sternothyroid and sternohyoid, anterior jugular veins.
- Posterior surface is related to 2nd to 4th tracheal rings.
- Superior border is related to the anastomosis between right and left superior thyroid artery
- Inferior thyroid artery leaves the inferior border.
Arterial supply:
- Superior thyroid artery, a branch of external carotid artery
- Inferior thyroid artery, a branch of thyrocervical trunk
- Arterio thyroidea ima, a branch of arch of aorta
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Venous drainage:
- Superior thyroid vein, drain into the external jugular vein
- Middle thyroid vein, drain into the internal jugular vein
- Inferior thyroid vein, drain into the internal jugular vein
- Fourth vein of kocher, drain into the external jugular vein.
Lymphatic drainage:
- Upper lobe drains into the upper deep cervical lymph node.
- Lower lobe drains into the lower deep cervical lymph node.
Nerve supply:
- Superior, middle and inferior cervical ganglion.
Clinical anatomy:
- Swelling of the thyroid is called GOITER.
- Hypothyroidism cause CRETINISM in infants and MYXOEDEMA in adults.
- Hyperthyroidism cause GRAVE’S DISEASE with and without thyrotoxicosis.
Inflammation of the thyroid gland is called THYROIDITIS
