L6.2 Thyroid

Question 1

Describe the position of the thyroid within the neck - what fascia is it within, underneath, and its internal shape

Answer 1

Thyroid gland is within the pretracheal fascia , sitting at level of 2nd and 3rd tracheal cartilages ~ around C6/7

It is underneath the thyroid cartilage shaped like a butterfly with two lateral lobes connected by the Isthmus (below cricoid cartilage).

Question 2

What does the length of the lateral lobes of the thyroid vary

Answer 2

Upper pole can go up to the level of thyroid cartilage, lower pole can go to the chest if a shorter neck - therefore to examine, may need to extend the neck

Question 3

Describe the development journey of the thyroid including relative times in weeks

Answer 3

1. At 3 weeks thyroid bud starts growing down from Foramen caecum at the base of the tongue as the thyroglossal duct.
2. During migration it travels down and passes under the hyoid bone
3. Finishes anteriorly to trachea in root of neck by 7 wks.

Question 4

What are some congenital disorders associated with thyroid development

Answer 4

1. Cysts,
2. fistulae/patent thyroglossal duct allowing food stuck/infection
3. ectopic thyroid tissue may persist in the midline and distinctively move with swallowing or protruding the tongue.
   eg. lingual (tongue) thyroid, cervical thyroid, pyramidal thyroid

Question 5

What are the arterial supplies to the thyroid, where does it originate and run

Answer 5

1. Superior thyroid arteries
   - Originate from Ext carotid artery
   - Run downward to reach upper pole of lateral lobe where it divides into ant and post glandular branches.
2. Inferior thyroid arteries
   - Originates from thyrocervical trunk (subclavian branch)
   - Runs upward behind carotid sheath to reach lower pole of lateral lobe where it divides into ascending and inferior branch,
3. Thyroid ima artery from the brachiocephalic trunk that supplies the isthmus

Question 6

Why is venous drainage of the thyroid important

Answer 6

1. In an operation its okay to ligate it, but you can’t leave it bleeding and sew someone back up
2. Depending on the position of a thyroid cancer, one drained by inferior thyroid goes to brachiocephalic whereas superior/middle to the int jugular.

Question 7

What is the venous drainage of the thyroid, where does it drain to

Answer 7

Superior, Middle and Inferior thyroid veins

Sup + Mid: Int Jugular vein

Inf: brachiocephalic veins

Question 8

What is the lymphatic drainage of the thyroid and what is clinically important about it

Answer 8

1. Paratracheal nodes
2. Deep cervical nodes along internal jugular vein (below omohyoid musc)
3. Some part of the upper thyroid may drain to lymph nodes along the base of the post triangle

Clinically if there is an enlarged lymph node in that region for more than 6 wks, painful/not painful refer to an ENT surgeon as it may be metastases

Question 9

What is the innervation of the thyroid gland

Answer 9

Autonomic system
1. Sympathetic trunk: sup, mid (at c3/4),inf cervical sympathetic ganglia

2. Parasympathetic:
   recurrent laryngeal nerve sitting posteriorly in tracheo-esophageal groove (from CNX).

Right goes under subclavian, Left under arch of aorta

Question 10

What are the tissue layers/spaces cut through from the skin to reach the thyroid- superior to deep

Answer 10

1. Skin
2. Subcutaneous fat with platysma
3. Investing layer
4. Infrahyoid muscles:
   a) Sternohyoid, b)Sternothyroid,
   c) thyrohyoid, d)omohyoid
5. Pretracheal space
6. Pretracheal fascia
7. Thyroid.

Question 11

Surgeons need to be aware of possible anatomical variations of thyroid when doing partial or full thyroidectomy because of what complications

Answer 11

• Carotid sheath with common carotid, int jugular and vagus is next to lateral lobes of thyroid
• Behind the thyroid is trachea, oesophagus: risk of fistula/ food going out
• Between them is - recurrent laryngeal nerve: damage to one = hoarseness of voice, two can lead to shut larynx- death
• inadvertent removal of parathyroid glands resulting in insufficiency - hypocalcaemia
• superior laryngeal nerve injury - damaging ext which lies deep to sup thyroid artery (or int branch). unilateral - difficulty in pitch changes, easy fatigability of voice