Thyroid
1) Embryology
a. Foramen cecum
i. Endodermal thickening devlopes in the floor of the primitive pharynx at the junction between the first & second pharyngeal pouches
ii. Diverticulum from foramen cecum descends anterior to hyoid bone and laryngeal cartilages and becomes thyroid
b. Thyroglossal duct
i. Temporary attachment between thyroid and foramen cecum that eventually disappears
ii. May leave reminants
1. pyramidal lobe
2. levator superioris thyroideae
2) Anatomy
a. Blood supply
i. Superior thyroid artery
1. first branch of external carotid
ii. Inferior thyroid artery
1. thyrocervical trunk from subclavian
iii. thyroidea ima artery
1. inconsistently present
2.
arises from
a. innominate artery
b. right subclavian artery
c. aortic arch
b. Venous drainage
i. Superior thyroid vein
1. drains to IJ
ii. Middle thyroid vein
1. drains to IJ
2. usually first vessel encountered during thyroidectomy
iii. Inferior thyroid vein
1. drains to innominate
c. Nerve supply (located in proximately to thyroid, don’t actually innervate it)
i. External laryngeal nerve
1. branch of superior laryngeal nerve
2. accompanies superior thyroid pedicle
3. innervated cricothyroid muscle
a. alters voice pitch if cut
ii. Recurrent Laryngeal nerve
1. branch of vagus
2. innervate vocal cords
3. arises in close relation to the fourth aortic arch
a. subclavian artery on right
b. aortic arch on left
4. because of descent of 4th aortic arches, recurrent nerve
a. drawn caudally into mediastinum
b. then course upward to final destination
5. usually lies in tracheoesophageal groove near terminal branches of inferior thyroid artery
6. Simon’s triangle
a. Usual location of nerve
b. Inferior thyroid artery superiorly, common carotid laterally, esophagus medially
7. <1.5% have non-recurrent recurrent laryngeal nerve
8. <0.5% with non-recurrent and recurrent laryngeal nerve
d. Parathyroids
i. Related to posterior aspect of each thyroid lobe