The thyroid gland

ByProf /gouda ellabban

The thyroid glandLobesPositionBlood supplyDevelopmentParathyroid glandsTracheostomy

Related topicPlan of the neck

The thyroid gland derives its name from the thyroid cartilage which resembles a shield

(G. thyreos = shield)

Function The thyroid gland is

an endocrine gland that is responsible for the secretion of thyroxin and thyrocalcitonin

Lobes The thyroid gland

consists of two lobes united in front of the second, third and fourth tracheal rings by an isthmus of gland tissue.

isthmus

Lobes Each lobe is pear-

shaped consisting of a narrow upper pole and a broader lower pole

upper pole

lower pole

Thyroid scan This nuclear scan uses

an injectable radioactive compound. When injected into the bloodstream the compound will be concentrated in the thyroid gland resulting in an image of the gland

The test can be useful in diagnosis of thyroid tumor

Position

It lies under cover of sternothyroid and sternohyoid muscles on the side of the larynx and trachea

ster

noth

yroi

d

ster

nohy

oid

Position

The upper pole of the thyroid cannot normally rise above the level of the oblique line of the thyroid cartilage

Thyroid, upper pole

sternothyroidthyrohyoidcricothyroid

The thyroid gland is caught in the pocket of sternothyroid

thyroidcr

icoid

thyr

oid

carti

lage

sternothyroid

thyrohyoid

cricothyroid

Position

The lower pole of the thyroid gland extends along the side of the trachea as low as the sixth tracheal ring

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Position

Because of the proximity of the thyroid gland to the trachea and esophagus, goiter causes compression of the trachea and esophagus resulting in dyspnea and dysphagia respectively

esophagus

Retro-sternal goitre with tracheal deviation

Retro-sternal goitre with esophageal deviation

Pyramidal lobe In about 40% of

people, there is a small upwards extension of the isthmus called the pyramidal lobe.

Levator glandulae thyroidae

The pyramidal lobe may be attached to the hyoid bone by fibrous or muscular tissue (levator glandulae thyroidae).

Variations Bifurcation of the

lower end of the pyramidal process, one part going to each lateral lobe

Variations Pyramidal process

attached to the left lobe of the gland, isthmus absent.

Variations Both pyramidal

process and isthmus are absent.

Pre-tracheal fascia The thyroid gland is

surrounded by a fibrous capsule and is enclosed in the pre-tracheal fascia

Pre-tracheal fascia The pre-tracheal

fascia attaches the thyroid gland to the trachea and larynx

thus the thyroid moves upwards on swallowing, an important diagnostic feature for lumps in the neck

thyroid

larynx

Blood supply The thyroid gland is

very vascular The vessels lie

between the capsule and the pre-tracheal fascia.

In some pathological conditions such as thyrotoxicosis, owing to its high vascularity, the blood flow can be heard with a stethoscope as a bruit

Thyroid arteries The main arteries

are the superior and inferior thyroid arteries.

superiorthyroid a.

inferiorthyroid a.

Superior thyroid artery

Arises from the anterior surface of the external carotid immediately distal to the carotid bifurcation.

externalcarotid a.

carotidbifurcation

Superior thyroid artery Arches downwards,

giving a sternomastoid branch and a superior laryngeal branch that enters the larynx with the nerve of the same name

superiorlaryngeala. & n.

Superior thyroid artery

enters deep to sternothyroid

ster

noth

yroi

d

Superior thyroid vessels

Superior thyroid artery before reaching the

upper pole of the gland, and within the pre-tracheal fascia, it divides into two main branches one for either surface of the gland

anterior posterior

Superior thyroid artery the posterior

branch anastomoses with the inferior thyroid artery

posterior br.of superiorthyroid a.

inferiorthyroid a.

Inferior thyroid artery Is a branch

of the thyrocervical trunk from the subclavian artery. subclavian a.

thyrocervicaltrunk

inferiorthyroid a.

Inferior thyroid artery Ascends and

turns medially at the level of the cricoid cartilage to enter the back of the gland some distance above the lower pole.

Inferior thyroid artery The tortuous course of

the inferior thyroid artery is due to the fact that in every swallow the thyroid gland ascends a few centimeters and must naturally drag its blood supply with it.

If this artery has no capability to elongate, it would be traumatized

Inferior thyroid arteryDivides outside

the pre-tracheal fascia into four or five branches that pierce the fascia separately to reach the lower pole of the gland.

Remember that the superior thyroid artery divides within the pretracheal fascia

The recurrent laryngeal nerve lies normally behind the branches of the inferior thyroid artery

The recurrent laryngeal nerve lies normally behind the branches of the inferior thyroid artery

but it is common for the nerve to pass between the artery branches before they pass through the fascia.

The recurrent laryngeal nerve always lies behind the pre-tracheal fascia and if this structure remains intact during thyroidectomy the nerve will not have been divided recurrent laryngeal n.

inferior thyroid a.

Both thyroid arteries are related to nerves which must be avoided when tying the arteries.

A little distance behind the superior thyroid artery is the external laryngeal nerve.

superior thyroid a.

external laryngeal n.

external laryngeal n.

internal laryngeal n.

superior laryngeal n.

Superior laryngeal nerve variationsvagus

internal

external

To avoid injury to the external laryngeal nerve, the superior thyroid artery is ligated and sectioned near the superior pole of the thyroid gland where it is not so closely related to the nerve as it is at its origin.

Section of the external laryngeal nerve produces weakness of voice, since the vocal fold cannot be tensed.

The cricothyroid muscle is paralyzedCricothyroid tenses the vocal cord

The recurrent laryngeal nerve has a variable relationship to the inferior thyroid artery

because of its proximity to the inferior thyroid artery and the pre-tracheal fascia it may be injured while ligating the artery during thyroidectomy

hence the advisability of ligating the inferior thyroid artery well lateral to the gland before it begins to divide into its terminal branches.

the inferior thyroid artery gives off esophageal and inferior laryngeal branches before its terminal distribution into the thyroid gland

site ofinferiorthyroid a.ligation

site ofsuperiorthyroid a.ligation

The variable relationship of the inferior thyroid artery to the recurrent laryngeal nerve makes thyroid surgery a potential risk to normal speech

The recurrent laryngeal nerve supplies all the intrinsic muscles of the larynx

it is advisable that a surgeon about to perform a thyroidectomy examines the vocal cords prior to operation, so that if there is any problem postoperatively one knows at least the origin of the lesion.

Recurrent laryngeal nerve damage

Is a complication of thyroid surgery that causes paralysis of the vocal cords

When bilateral the voice is almost absent as the two vocal folds cannot be adducted.

Recurrent laryngeal nerve damage

A unilateral recurrent laryngeal nerve injury may not be noticed in normal speech but would be very detrimental to a singers career.

The thyroid arteries anastomose freely with each other and with tracheal and esophageal arteries.

In operations of partial or sub-total thyroidectomy, all four arteries are tied

In operations of partial or sub-total thyroidectomy, all but the posterior part of the gland excised

remainingthyroidtissue

the dangerous anatomy lies in the posterior lateral lobes (recurrent laryngeal nerve and the parathyroid glands)

Recurrentlaryngeal n.

parathyroidgland

The remains of the gland are located alongside the trachea and contain the parathyroid glands, the whole being supplied with blood by the anastomosis

Thyroidae ima artery In about 10% of

individuals, an unpaired artery, the thyroidae ima (L. ima = lowest) is a small occasional artery from the brachiocephalic trunk, or left common carotid artery, or direct from the arch of the aorta

Thyroidae ima artery Ascends anterior to

trachea and supplies the isthmus of the thyroid gland.

Thyroidae ima artery The possible presence

of the thyroid ima artery must be remembered when incising the trachea inferior to the isthmus.

As the thyroidae ima runs anterior to the trachea, it is a potential source of serious bleeding

Thyroid veins The veins are three

in number on each side

the superior thyroid vein from the upper pole follows the artery and enters the internal jugular vein or the common facial vein

Superior thyroid v.

Internal jugular v.

The middle thyroid vein is short and wide, it enters the internal jugular vein

Thyroid veins

middle thyroid v.

Internal jugular v.

From the isthmus and lower pole of the gland the inferior thyroid veins form a plexus within the pre-tracheal fascia that descends in front of the trachea to reach the left brachiocephalic vein

Thyroid veins

inferior thyroid vv.

brachiocephalic v.

As the inferior thyroid veins cover the anterior surface of the trachea inferior to isthmus, they are potential sources of bleeding during tracheotomy (also remember the situation of the thyroidae ima artery).

Inferior thyroid veins

Development of the thyroid gland

The gland begins as a diverticulum from the floor of the embryonic pharynx

Development of the thyroid gland

The diverticulum grows caudally superficial to the hyoid before dividing into two lobes

The stem of the diverticulum, the thyroglossal duct, normally disappears

hyoid

Thyroglossal duct

Development of the thyroid gland

After the tongue has developed, it can be seen that the point of outgrowth of the thyroglossal duct is the foramen cecum (of Morgagni) [Morgagni, Giovanni Battista, 1682-1771, a Padua anatomist and pathologist, also known for hydatid of Morgagni (appendix testis) and anal columns (of Morgagni)].

Thyroglossal cyst cysts derived from

the duct may also appear anywhere between the foramen cecum and the normal position in the midline of the neck1. Beneath foramen cecum2. Floor of the mouth3. Suprahyoid4. Subhyoid5. On thyroid cartilage6. At level of cricoid cartilage

Thyroglossal cyst Can be diagnosed

because characteristically it moves upwards as the patient puts his tongue out.

Infection of a thyroglossal cyst may spread to a persistent thyroglossal duct which must be then excised

Although the duct lies ventral to the hyoid bone, it passes up for a short distance behind the body, which therefore has to be excised with the duct

Accessory thyroid gland Aberrant thyroid

tissue may appear between the foramen cecum and the normal position

Lingual thyroid

Rarely the thyroid fails to descend during development resulting in the development of a lingual thyroid

Ectopic thyroid

Failure of descent mar result in a superior cervical thyroid in the region of the hyoid bone

the thyroid may sometimes descended too far and be found in the superior mediastinum

Parathyroid glands Two on each side They are yellow-brown

endocrine glands, about the size of a small pea (about 0.5x0.8 cm ovoids)

They are important because of their role in calcium metabolism. They secrete parathormone that mobilizes bone calcium and increases gut and kidney calcium absorption

Parathyroid glands Are located

posterior to the thyroid gland between its capsule and fascial sheath

Superior parathyroid glands

more constant in position

embedded in the posterior surface of the thyroid gland, a short distance above the entry of inferior thyroid artery (and the level of the cricoid cartilage).

Inferior parathyroid glands variable in position usually embedded

behind the lower pole but is often found elsewhere (they may even present in the superior mediastinum).

Para

thyr

oid

deve

lopm

ent

The parathyroids develop from the endoderm of the third (inferior gland) and fourth (superior gland) pharyngeal pouches

The thymus also develops from the third pouch and may therefore carry the inferior parathyroid with it when it descends into the thorax.

Para

thyr

oid

deve

lopm

ent

Parathyroid glands, blood supply

The glands are usually supplied by the inferior thyroid arteries but may also be supplied by both superior and inferior thyroid arteries

posterior br.of superiorthyroid a.

inferiorthyroid a.

Parathyroid glands Awareness of the

close relationship between the parathyroid glands and the thyroid gland is essential to prevent removal or damage of the parathyroid glands during thyroidectomy.

The parathyroid glands are usually safe during subtotal thyroidectomy because the posterior part of the thyroid gland is preserved

The variability in position of the parathyroid glands may create a problem during total thyroidectomy; in this case the parathyroid glands are saved by following their small vessels which are kept intact before the thyroid is removed.

When tracheostomy is done electively after establishing an airway with an endotracheal tube, a short transverse incision is made one cm below the cricoid cartilage

Tracheostomy

Endotracheal tube

Tracheostomy The transverse

incision is made midway between the cricoid cartilage and the sternal notch

Tracheostomy The decussating

fibers of platysma are divided.

Tracheostomy After elevating

platysma, the investing fascia between the strap muscles is incised

Tracheostomy The pretracheal

(strap) muscles are seperated

Tracheostomy The pretracheal

fascia is split longitudinally

The thyroid isthmus is divided and sutured

The second tracheal ring is precisely identified and divided vertically in the midline, extending the incision through the third ring in most cases

The first ring is preserved

Tracheostomy

A thyroid retractor gently spreads the tracheal opening.

The tracheostomy tube with obturator is introduced after withdrawing the endotracheal tube under direct vision to a point just above the stoma

Tracheostomy

retractor

Tracheostomy tube

Endotracheal tube

Tracheostomy If more room is

needed, the fourth ring may be partially divided

A transverse incision is to be avoided.

The skin is closed loosely

The flange of the tracheostomy tube not only is tied with a tape around the neck but also is sutured to the skin.

Tracheostomy tube flange

4th tracheal ring

Tracheostomy The endotracheal tube

is removed only when the tracheostomy tube has been shown to provide a satisfactory airway

If there is any question about where the tip of the tube lies, a flexible bronchoscope may be used to check the distal position.

The tracheostomy tube should be just large enough to provide an adequate airway for the patient. Larger tubes can only cause damage.

It must be remembered that most women, even when obese, have tracheas smaller in diameter than those of men

Tracheostomy

Permanenttracheostomyopening

Complications of tracheostomy

the anterior jugular veins may be encountered as the superficial fascia is incised

They are avoided by maintaining a midline position

Complications of tracheostomy

Sometimes a large jugular venous arch may be encountered

Complications of tracheostomy

The inferior thyroid veins are often asymmetric, hence more liable to injury

Complications of tracheostomy

The branches of the superior and inferior thyroid arteries may anastomose across the midline

Complications of tracheostomy

A thyroid ima artery is very occasionally present and must be ligated if found

Complications of tracheostomy

The brachiocephalic artery and vein may be injured if sharp dissection is carried too far downwards

The artery may be eroded by a tracheostomy tube, resulting in a tracheo-arterial fistula

Complications of tracheostomy

In children the left brachiocephalic vein and the thymus may extend above the suprasternal notch.

Complications of tracheostomy

The subclavian artery and vein may be compromised by a tracheostomy to that is incorrectly curved or is placed too low

Tube too curved Tube too low

Complications of tracheostomy

The existence of fascial planes predisposes to surgical emphysema, particularly if the skin is sutured too tightly.

Investing fascia

Complications of tracheostomy

Surgical emphysema may extend into the mediastinum. Investing

fasciapretrachealfascia

Complications of tracheostomy

Beware of over-enthusiastic incision into the trachea; the esophagus is immediately posterior.

trachea

esophagus

Thyroid & pretracheal fasciaInvesting fascia

Skin & superficial fascia