Thyroid
THYROID GLAND
The thyroid is an endocrine gland with a rich blood supply, situated in the lower part of the front and sides of the neck.
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Thyroid Gland
- The thyroid (shield-like, Greek) is an endocrine gland with a rich blood supply.
- It is situated in the lower part of the front and sides of the neck.
- Since it is placed superficially, it can easily be examined.
Functions
- It secretes tri-iodothyronine (T3), tetra-iodothyronine (T4), thyroxine and calcitonin.
- It regulates the basal metabolic rate.
- It promotes the psychosomatic growth of the body.
- It modulates the calcium metabolism.
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Parts of Thyroid Gland
The gland is H-shaped and consists of:
- Two lobes — right and left
- Isthmus — connects both lobes
- Pyramidal lobe — may project upwards from the isthmus or from one of the lobes.
Levator Glandulae Thyroideae
- It is a fibrous or fibromuscular band that is sometimes present.
- It connects the pyramidal lobe to the body of the hyoid bone.
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Location and Extent
- The gland lies against vertebrae C5–C7 and T1, embracing the upper part of the trachea.
- Each lobe extends from the middle of thyroid cartilage to the 4th or 5th tracheal rings.
- The lobe is prevented from ascending by the thyrohyoid muscle.
- It is prevented from descending down by the ligament of Berry.
- The isthmus extends from the 2nd to the 4th tracheal ring.
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Dimensions and Weight
Each Lobe
5 × 2.5 × 2.5 cm
Isthmus
1.2 × 1.2 cm
Average Weight
25 g
Variation
- The thyroid gland is larger in females than in males.
- It further increases in size during menstruation and pregnancy.
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Capsules of Thyroid
The thyroid gland has two capsules:
1. Inner True Capsule
It is the peripheral condensation of the connective tissue of the gland.
2. Outer False Capsule
It is derived from the pretracheal layer of the deep cervical fascia.
Features of False Capsule
- It is thin along the posterior border of the lateral lobe.
- It is thicker on the medial surface of the lateral lobe.
Suspensory Ligament of Berry
- It is a thickened part of the pretracheal fascia (false capsule).
- It connects the lobes of thyroid gland with the cricoid cartilage.
Clinical Anatomy — Capsules
- 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.
- This can be compared with the prostate, in which the venous plexus lies between the two capsules of the gland.
- Therefore, during prostatectomy, both capsules are left behind.
Comparison with Prostate
- In prostatic surgeries, the venous plexus lies between two capsules.
- Hence, the prostate should be removed, leaving behind both capsules.
Swelling of Thyroid
As the capsule of the thyroid gland is thin along the posterior border, its swelling more commonly extends backward rather than forming a visible swelling in the neck.
Swallowing and Thyroid Movements
- The thyroid moves up and down during swallowing.
- This occurs because it is attached to the cricoid cartilage by the ligament of Berry.
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Parts and Relations of Lobes
The lobes are conical in shape and have:
1. Apex
2. Base
3. Three Surfaces
- Lateral
- Medial
- Posterolateral
4. Two Borders
- Anterior
- Posterior
A. Apex
- It is directed upwards and slightly laterally.
- It is limited superiorly by the attachment of the sternothyroid muscle to the oblique line of thyroid cartilage, which is medial to the apex.
- Thus, it lies between the:
- Inferior constrictor
- Sternothyroid muscles
The apex is related to:
- Superior thyroid artery
- External laryngeal nerve
B. Base
- It is at the level of the 4th or 5th tracheal ring.
- It is related to:
- Inferior thyroid artery
- Recurrent laryngeal nerve
C. Lateral or Superficial Surface
- It is convex.
- It is covered by:
- Sternohyoid
- Superior belly of omohyoid
- Sternothyroid
- Anterior border of sternocleidomastoid
D. Medial Surface
It is related to:
Two Tubes
- Trachea
- Oesophagus
Two Muscles
- Inferior constrictor
- Cricothyroid
Two Nerves
- External laryngeal
- Recurrent laryngeal
Two Cartilages
- Thyroid
- Cricoid
E. Posterolateral Surface
- It is related to the carotid sheath.
- It overlaps the common carotid artery.
F. Anterior Border
- It is thin.
- It is related to the anterior branch of superior thyroid artery.
G. Posterior Border
- It is thick and rounded.
- It separates the medial and posterior surfaces.
- It is related to:
- Inferior thyroid artery
- Anastomosis between the posterior branch of superior thyroid artery and ascending branch of inferior thyroid artery
- Parathyroid glands
- Thoracic duct — only on the left side.
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Parts and Relations of Isthmus
The isthmus connects the lower parts of the two lobes.
It has:
- Two surfaces — anterior and posterior
- Two borders — superior and inferior
A. Anterior Surface
It is covered by:
- Right and left sternothyroid and sternohyoid muscles
- Anterior jugular veins
- Fascia and skin
B. Posterior Surface
- It is related to the 2nd to 4th tracheal rings.
C. Upper Border
- It is related to the anterior branches of the right and left superior thyroid arteries, which anastomose here.
D. Lower Border
- Inferior thyroid veins leave the gland at this border.
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Arterial Supply
The thyroid gland is supplied by:
- Superior thyroid arteries
- Inferior thyroid arteries
1. Superior Thyroid Artery
- It is the 1st 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, the artery pierces the pretracheal fascia to reach the apex of the lobe, where the nerve deviates medially.
At the Upper Pole
a. Anterior Branch
- Descends on the anterior border of the lobe.
- Continues along the upper border of the isthmus.
- Anastomoses with its counterpart on the opposite side.
b. Posterior Branch
- Descends on the posterior border of the lobe.
- Anastomoses with the ascending branch of inferior thyroid artery.
2. Inferior Thyroid Artery
- It is a branch of the thyrocervical trunk.
- The thyrocervical trunk arises from the subclavian artery.
- It runs:
- First upwards
- Then medially
- Finally downwards to reach the base of the gland
- It gives off branches to adjacent structures.
- Its terminal part is intimately related to the recurrent laryngeal nerve.
- Its 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. Lowest Thyroid Artery
- Sometimes, in 3% of individuals, the thyroid is also supplied by the lowest thyroid artery (thyroidea ima artery).
- It arises directly from the arch of the aorta.
- It enters the lower part of the isthmus.
4. Accessory Thyroid Arteries
Accessory thyroid arteries arising from tracheal and oesophageal arteries also supply the thyroid.
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Venous Drainage
The thyroid is drained by:
- Superior thyroid vein
- Middle thyroid vein
- Inferior thyroid veins
1. Superior Thyroid Vein
- Emerges at the upper pole.
- Accompanies the superior thyroid artery.
- Ends in the internal jugular vein.
2. Middle Thyroid Vein
- It is a short, wide channel.
- Emerges at the middle of the lobe.
- Soon enters the internal jugular vein.
3. Inferior Thyroid Veins
- Emerges at the lower border of the isthmus.
- Form a plexus in front of the trachea.
- Drain into the left brachiocephalic vein.
4. Fourth Thyroid Vein (Kocher)
- May emerge between the middle and inferior veins.
- Drains into the internal jugular vein.
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Lymphatic Drainage
Upper Part of Gland
Lymph reaches the upper deep cervical lymph nodes:
- Either directly
- Or through the prelaryngeal nodes
Lower Part of Gland
Lymph drains to the lower deep cervical nodes:
- Directly
- Also through the pretracheal and paratracheal nodes
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Nerve Supply
- Nerves are derived mainly from the middle cervical ganglion.
- They are partly derived from the superior and inferior cervical ganglia.
- These are vasoconstrictor nerves.
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Clinical Anatomy
Thyroid Swelling and Examination
- Any swelling of the thyroid gland (goitre) should be palpated from behind.
Thyroidectomy
- 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.
- It also avoids the risk of postoperative myxoedema, caused by deficiency of thyroid hormones.
Thyroidectomy and Arteries
- During thyroidectomy, the superior thyroid artery is ligated near the gland to save the external laryngeal nerve.
- The stem of inferior thyroid artery is not ligated.
- Its glandular branches are ligated separately.
- In this way, the blood supply to parathyroid glands is maintained.
Hypothyroidism
Hypothyroidism causes:
- Cretinism in infants
- Myxoedema in adults.
Benign Tumours of Thyroid Gland
Benign tumours of the gland may displace and even compress neighbouring structures:
- Trachea → causing dyspnoea (difficulty in breathing)
- Oesophagus → causing dysphagia (difficulty in swallowing)
- Recurrent laryngeal nerve → dysphonia (hoarseness of voice).