Thyroid gland (dave's notes) Flashcards
Describe the shape and location of the thyroid gland
Two symmetrical lobes united in front of the 2nd, 3rd, and 4th tracheal rings by a glandular isthmus
What is the shape of the lateral lobes of the thyroid?
Pear or triangular in shape with a narrow upper pole and a broad lower pole
What is the shape of the lateral lobe of the thyroid in cross section? What surfaces does it have?
Triangular, with lateral, medial and posterior surfaces
What covers the lateral surface of the lateral lobe of the thyroid?
Sternothyroid and sternohyoid
What is in contact with the medial surface of the lateral lobe of the thyroid?
The lateral side of the larynx and upper trachea
How far inferior may the lower pole of the thyroid extend?
The 6th tracheal ring.
What are the medial muscular relations of the thyroid?
Cricothyroid and inferior constrictor muscles
What nerve approaches the thyroid from above? What nerve from below?
External laryngeal nerves approach from above
Recurrent laryngeal nerve from below
What does the posterior surface overlap? What may happen in pathological enlargement here?
The posterior surface overlaps the medial part of the carotid sheath and can extend past the internal jugular vein in enlargement
Where are the parathyroid glands usually found?
In contact with the posterior surface of the lateral lobes of the thyroid
Via what route to the recurrent laryngeal nerves approach the thyroid? How does the left differ from the right RLN?
The recurrent laryngeal nerves approach the medial surface of the gland from below, just in front of the trachea-oesophageal groove. The left is more likely to lie behind the inferior thyroid artery. On the right there is an equal chance.
What is the recurrent laryngeal nerve always found behind?
The pretracheal fascia and behind the cricothyroid joint; here it passes upwards under inferior constrictor
What may happen to the recurrent laryngeal nerve at the upper border of the isthmus?
It may divide into two. The anterior branch will always be motor.
How does the external laryngeal nerve approach the thyroid?
It runs down to supply cricothyroid behind the superior thyroid artery
Describe the thyroid isthmus
The thyroid isthmus joins the anterior surfaces of the left and right thyroid lobes and is firmly adherent to the 2nd, 3rd, and 4th tracheal rings.
What runs across the thyroid isthmus?
An arterial anastomosis.
Where may a pyramidal lobe be found? What does this represent?
A pyramidal lobe may be found projecting forwards on the left representing the caudal end of the thyroglossal duct.
What other developmental remnant may be found?
An accessory remnant may also be found
What is the arterial supply to the thyroid?
Superior thyroid
Inferior thyroid
Thyroidea ima artery
From what does the superior thyroid arise? What branches does it give off first?
The first branch of the anterior aspect of the external carotid.
Gives off SCM and superior laryngeal branches before piercing the pretracheal fascia to reach the summit of the upper pole.
What is found immediately behind the superior thyroid artery?
The external laryngeal nerve. The artery is divided right at the pole.
From what does the inferior thyroid artery arise? What branches does it give off first?
The inferior thyroid artery arises from the thyrocervical trunk. It arches upwards behind the lower pole to divide outside the pretracheal fascia into 4-5 branches that pierce separately.
Gives of oesophageal and inferior laryngeal branches first.
In thyroidectomies is ligated well lateral.
Where is the thyroidea ima artery found? From which arteries may it originate?
The thyroidea ima artery enters the lower part of the isthmus in 3% of individuals. It can arise from the brachiocephalic trunk, the right common carotid, or the aortic arch.
Describe the venous drainage of the thyroid
Venous return from the upper pole follows the superior thyroid artery (superior thyroid vein) to the internal jugular vein or the FV (??).
The middle thyroid vein passes from the middle directly to the IJV.
Inferior thyroid veins form a plexus which drains primarily into the left brachiocephalic vein.
Where does thyroid lymph drain to?
Lymph drainage follows the arteries.
The upper pole drains to the anterosuperior group, and the lower pole to the postero-inferior group.
Describe the nerve supply to the thyroid
The bulk of sympathetic supply is from the middle cervical ganglion, fibres of which enter on the inferior thyroid artery. A few vagal filaments also exist.
What percentage of cells are parafollicular? What do they secrete?
2%, Calcitonin
Describe the histological structure of the thyroid
The thyroid has a mass of rounded follicles full of colloid (iodine containing product of epithelial cells). Colloid is iodinated within the cells, stored outside, then reabsorbed for transport
Summarise the development of the thyroid
The thyroid develops as a proliferation of cells from the caudal end of the thyroiglossal duct. Parafollicular calcitonin cells come from the 5th pharyngeal pouch, but no other part does.
