Thyroid Gland

Features:

• It is an endocrine gland 
• Situated in the lower part of front & sides of neck 
• Regulates the basal metabolic rate 
• stimulates somatic & psychic growth 
• plays important role in calcium metabolism 
• uses natural iodine for the synthesis of hormone. 
• hormones stored within the follicles & used according to the needs of the body

• It has - Right & left lobes, joined by isthmus
• A third lobe (pyramidal Lobe) may project upwards from isthmus
• Sometime a fibrous or fibre-muscular band ( levator glandulae thyroidae) descends from the body of hyoid bone to the isthmus or to the pyramidal lobe. 

Situation 

• Lies against C5-C7 & T1vertebrae
• embracing the upper part of trachea 

Extent 

• Each lobe extends from middle of thyroid cartilage to 4th or 5th tracheal ring
• Isthmus extends from 2nd to 4th tracheal ring 

Dimensions 

• lobe - 5 x 2.5 x2.5 cms 
• isthmus - 1.2 x 1.2 cms

Weight 

• 25gms 
• Larges in females than males 
• increases in size during menstruation & pregnancy

Capsules of thyroid gland 

1. True capsule - 

• formed by peripheral condensation of connective tissue of the gland.
• A dense capillary plexus is present deep to the true capsule.
• To avoid haemorrhage during operations, the thyroid is removed along with the true capsule.
• It can be compared with the prostate in which the venous plexus lies between the two capsules of the gland; and, therefore, during prostatectomy both capsules are left behind.

2. False capsule - 

• formed by pre-tracheal layer of deep cervical fascia 
• It 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 lobe to the cricoid cartilage

Lateral lobe

The lobes are conical in shape having:

• a. An apex
• b. A base
• c.Three Surfaces: Lateral, medial and posterolateral.
• d.Two Borders: Anterior and posterior.

• The apex is directed upwards and slightly laterally.
• It is limited superiorly by the attachment of the sternothyroid to the oblique line of thyroid cartilage. 
• The apex is related to superior thyroid artery and external laryngeal nerve 
• The base is at level with the 4th or 5th tracheal ring.
• It is related to inferior thyroid artery and recurrent laryngeal nerve

The lateral or superficial surface is convex, and  is covered by:

• a. The sternohyoid
• b. The superior belly of the omohyoid
• c. The sternothyroid
• d. The anterior border of the sternocleidomastoid

The medial surface is related to:

• a. Two tubes, trachea and oesophagus
• b. Two muscles, inferior constrictor and cricothyroid
• c. Two nerves, external laryngeal and recurrent laryngeal

The posterolateral or posterior surface is related to the carotid sheath and overlaps the common carotid artery

The anterior border is thin and is related to the anterior branch of superior thyroid artery.

The posterior border is thick and rounded and separates the medial and posterior surfaces. 

It is related to:

• a. Inferior thyroid artery.
• b. Anastomosis between the posterior branch of superior and ascending branch of inferior thyroid arteries.
• c. Parathyroid glands.
• d. Thoracic duct only on the left side

The isthmus connects the lower parts of the two lobes.

It has:

• a. Two surfaces: Anterior and posterior.
• b. Two borders: Superior and inferior.

The anterior surface is covered by:

• a. The right and left sternothyroid and sternohyoid muscles.
• b. The anterior jugular veins.
• c. Fascia and skin

The posterior surface is related to the second to fourth tracheal rings.

The upper border is related to anterior branches of the right and left superior thyroid arteries which anastomose here.

The Lower border - Inferior thyroid veins leave the gland at this border

Arterial Supply

• The thyroid gland is supplied by the superior and inferior thyroid arteries.

1. The superior thyroid artery is the first anterior branch of the external carotid artery

• It runs downwards and forwards in intimate relation to the external laryngeal nerve. 
• After giving branches to adjacent structures, it pierces the pretracheal fascia to reach the upper pole of the lobe where the nerve deviates medially.
• At the upper pole the artery divides into anterior and posterior branches.
• The anterior branch descends on the anterior border of the lobe and continues along the upper border of the isthmus to anastomose with its fellow of the opposite side.
• The posterior branch descends on the posterior border of the lobe and anastomoses with the ascending branch of inferior thyroid artery.

2. The inferior thyroid artery is a branch of thyrocervicaltrunk (which arises from the subclavian artery)

• It runs first upwards, then medially, and finally downwards to reach the lower pole of the gland.
• During its course, it passes behind the carotid sheath and the middle cervical sympathetic ganglion; and in front of the vertebral vessels; and gives off branches to adjacent structures.
• Its terminal part is intimately related to the recurrent laryngeal nerve, while proximal part is away from the nerve.
• The artery divides into 4 to 5 glandular branches which pierce the fascia separately to reach the lower part of the gland.
• One ascending branch anastomoses with the posterior branch of the superior thyroid artery, and supplies the parathyroid glands.

3. Sometimes (in 3% of individuals), the thyroid is also supplied by the lowest thyroid artery (thyroidea ima artery) which arises from the brachiocephalic trunk or directly from the arch of the aorta. 

• It enters the lower part of the isthmus. 

4. Accessory thyroid arteries arising from tracheal and oesophageal arteries also supply the thyroid.

Venous Drainage

The thyroid is drained by the superior, middle and inferior thyroid veins.

• The superior thyroid vein emerges at the upper pole and accompanies the superior thyroid artery. It ends in the internal jugular vein
• The middle thyroid vein is a short, wide channel which emerges at the middle of the lobe and soon enters the internal jugular vein.
• The inferior thyroid veins emerge at the lower border of isthmus. They form a plexus in front 
• of the trachea, and drain into the left brachiocephalic vein.
• A fourth thyroid vein (Kocher) may emerge between the middle and inferior veins, and drain into the internal jugular vein.

Lymphatic Drainage

• Lymph from the upper part of the gland reaches the upper deep cervical lymph nodes either directly or through the prelaryngeal nodes. 
• Lymph from the lower part of the gland drains to the lower deep cervical nodes directly, and also through the pretracheal and paratracheal nodes.

Nerve Supply

• Nerves are derived mainly from the middle cervical ganglion and partly from the superior and inferior cervical ganglia. 
• These are vasoconstrictor.

HISTOLOGY

The thyroid gland is made up of the following two types of secretory cells.

I. Follicular cells lining the follicles of the gland secrete tri-iodothyronin and tetraiodothyronin (thyroxin)

which stimulate basal metabolic rate and somatic and psychic growth of the individual. 

During active phase, the lining of the follicles is columnar, while in resting phase, it is cuboidal. 

Follicles contain the colloid in their lumina.

2 Parafollicular cells (C cells) are fewer and light cells lie in between the follicles. 

They secrete thyrocalcitonin which promotes deposition of calcium salts in skeletal ind other tissues, and tends to produce hypocalcaemia. 

These effects are opposite to those of parathormone.

DEVELOPMENT

• The thyroid develops from a median endodermal thyroid diverticulum which grows down in front of 
• the neck from the floor of the primitive pharynx, just caudal to the tuberculum impar.
• The lower end of the diverticulum enlarges to formthe gland. 
• The rest of the diverticulum remains narrow and is known as the thyroglossal duct.
• Most of the duct soon disappears. 
• The position of the upper end is marked by the foramen caecum of the tongue, and the lower end often persists as the pyramidal lobe. 
• The gland becomes functional during third month of development.

• Remnants of the thyroglossal duct may form thyroglossal cysts, or a thyroglossal fistula. 
• Thyroid tissue may develop at abnormal sites along the course of the duct resulting in lingual or 
• retrosternal thyroids.
• Accessory thyroids may be present.

CLINICAL ANATOMY

• Any swelling of the thyroid gland (goitre) should be palpated from behind.
• Removal of the thyroid (thyroidectomy) with true capsule may be necessary in hyperthyroidism.
• In subtotal thyroidectomy, the posterior parts of both lobes are left behind. This avoids the risk of simultaneous removal of the parathyroids and also of postoperative myxoedema.
• During thyroidectomy, the superior thyroid artery is ligated near the gland to save the external laryngeal nerve; and the inferior thyroid artery is ligated away from the gland to save the recurrent laryngeal nerve.
• Hypothyroidism causes cretinism in infants and myxoedema in adults.
• Benign tumours of the gland may displace and even compress neighbouring structures, like the carotid sheath, the trachea, etc. 
• Malignant growths tend to invade and erode neighbouring structures.
• Pressure symptoms and nerve involvements are common in carcinoma of the glands.

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Thyroid Gland |Parts| Relations| Blood supply|Microscopic structure | Development | Clinical Anatomy