Anatomy/Physiology Thyroid

Question 1

Thyroid originates from:

a) endoderm
b) mesoderm
c) ectoderm

Answer 1

A) endoderm

Question 2

Describe developmental embryology of thyroid gland.

Answer 2

• midline diverticulum arises in foramen cecum at base of tongue at 4 weeks gestation
• fibrous tract becomes thyroglossal duct (including portion associated with hyoid bone)
• distal path of descent-may be retained as pyramidal lobe

Question 3

What is the thyroglossal duct?

Answer 3

embryological anatomical structure forming open connection between initial area of thyroid gland and final position

Question 4

How do you get a lingual thyroid gland?

What is treatment for this?

Answer 4

failure of median thyroid component to descend from foramen cecum

Treatment: radioactive iodine ablation

Question 5

How does thyroglossal duct cyst form?

Answer 5

persistent thyroglossal duct

occurs in midline of neck along path of descent of median thyroid component

-may occur from base of tongue to low central neck, although most located below hyoid bone

Question 6

What is treatment for thyroglossal duct cyst?

Answer 6

Sistrunk procedure

-excise entire thyroglossal duct tract and a chunk of the hyoid bone

Question 7

Any malignant potential with thyroglossal duct?

Answer 7

epithelial lining of cyst contains some thyroid cells and some carcinomas (85% papillary type, also Hurthle, follicular, but never medullary)

Question 8

What is an ectopic thyroid? What are some common locations?

Answer 8

thyroid tissue found other than at usual anatomic location

Ectopic thyroid is usually located along the normal path of thyroid gland descent but rarely can also be found in mediastinum, heart, esophagus, or diaphragm

Ectopic thyroid tissue is derived from abnormalities in migration of the medial anlage and hence typically does not contain C cells.

Question 9

What is thyrothymic tract rest?

Answer 9

Tissue that arises in thyrothymic tract below thyroid lobes

Question 10

What is Ligament of Berry? Significance?

Answer 10

lateral thyrohyoid ligament

internal branch of superior laryngeal nerve lies lateral to it

Question 11

Arterial supply of thyroid gland?

Answer 11

Superior thyroid artery - branches off from external carotid artery

Inferior thyroid artery- branches off from thyrocervical trunk which comes off right subclavian

Occasionally have thyroid ima artery- may arise from innominate artery or aortic arch

Question 12

Venous drainage of thyroid gland?

Answer 12

superior thyroid veins - empties into internal jugular (IJ) veins

middle thyroid veins- present in more than 50% of patients, drain laterally into IJ

inferior thyroid veins- empties into right (brachiocephalic) and left innominate veins

Question 13

Branches off right subclavian artery?

Answer 13

VIT (vitamin)

V- vertebral

I- internal mammary

T- thyrocervical trunk

Question 14

Branches off thyrocervical trunk?

Answer 14

A-SIT

A- ascending cervical

S- suprascapular

I- inferior thyroid

T- transverse cervical (present in only 30% of cases)

Question 15

Where do the RLN and SLN arise?

Answer 15

off vagus

Question 16

What is the course of the vagus nerve?

Answer 16

arises from medulla in brainstem and exits skull via jugular foramen

-sits in carotid sheath between internal jugular vein and internal carotid artery

Question 17

Where do the right and left recurrent laryngeal nerves swing around?

Answer 17

Right RLN- hooks under right subclavian

Left RLN- hooks under aortic arch

passes inferior and medial to arch and ascends to larynx in TE groove

“The left vagus nerve left you under the arch and slid up the groove.”

Question 18

What is a non-recurrent laryngeal nerve?

What % of the population has this?

Answer 18

arises from vagus and passes directly into larynx

anomaly more common on right (0.5-0.7%) compared to left (0.04%)

can be mistaken fro inferior thyroid artery -mistakenly divided

Question 19

What are the functions of the RLN?

Answer 19

mixed motor, sensory and autonomic functions

• innervates intrinsic laryngeal muscles (except cricothyroid muscle)
• motor fibres to cervical segments of esophagus and trachea
• sensory fibres- vocal cords

Question 20

What is the tubercle of Zuckerkandl?

Answer 20

posterior extension of lateral lobe of thyroid gland

• covers RLN
• tubercle often called posterior horn
• RLN often found lateral to tubercle

Question 21

What is the superior laryngeal nerve?

Answer 21

-arises from vagus at nodose ganglion near skull, descends along course of internal carotid artery

internal branch- enters larynx at thyrohyoid membrane - sensory innervation to superior larynx

external branch- motor innervation to cricothyroideus muscle (changes pitch of voice)

if bilateral injury- hoarse voice, changes projection of choice

-can be injured when taking superior thyroid arteries

Question 22

Where do most thyroid cancers drain?

Answer 22

Level VI (central compartment)

Question 23

What are skip metastases in thyroid gland?

Answer 23

most thyroid cancers drain directly to Level VI (central compartment) except for cancers in superior third of gland, which drain to lateral compartment

Question 24

Lymph node compartments?

Answer 24

Level I- submental

Level II- upper jugular

Level III- mid jugular

Level IV- lower jugular

Level V- posterior to SCM (posterior triangle)

Level VI- central compartment

Level VII- superior mediastinal

Question 25

What is the relationship of the parathyroid glands to the recurrent laryngeal nerve?

Answer 25

AI (inferior parathyroids- anterior to RLN), artificial intelligence

PS I love you: (Superior parathyroids- posterolateral to RLN)

Question 26

What are the common locations of ectopic parathyroid glands?

Answer 26

Thyroid parenchyma- 33%

Thyrothymic ligament- 19%

Thymus- 15%

Carotid artery sheath- 7%

Mediastinum- 7%

Question 27

What are the strap muscles?

Answer 27

TOSS

T- thyrohyoid

O- omohyoid

S- sternothyroid (posterior layer)

S- sternohyoid (anterior layer)

Question 28

Describe basic thyroid unit

Answer 28

Follicular unit- colloid at center, each follicle is surrounded by single layer of follicular cells

colloid filled mostly with thyroglobulin (protein in which T4/T3 are synthesized and stored)

parafollicular or C-cells: generate calcitonin

Question 29

How does T4 and T3 come about?

Answer 29

T3- most in circulation is derived from conversion of T4 to T3

in circulation, 80% of T3/T4 bound to thyroxine binding globulin

Thyroid produces 80% T4, 20% T3

Question 30

What are the half lives of T3 and T4?

Answer 30

half life T3 = 8-12 hours

half life T4 = 7 days

That’s why after thyroidectomy you need to stay on beta blocker for about two weeks to avoid effects of residual circulating T4 which can last up to 7-10 days after thyroid removed

Question 31

What is thyroglobulin?

Answer 31

glycoprotein in follicular cells- facilitates conversion of MIT and DIT into T3 and T4

of circulating T3 and T4, 80% bound to thyroxine binding globulin in periphery

Thyroglobulin reflects three factors:

1) mass of thyroid tissue present
2) presence of injury or inflammation of gland (allows leakage of Tg)
3) degree of stimulation of TSH receptor

Question 32

Whtat is calcitonin?

What stimulates it?

Answer 32

secreted by parafollicular (C-cells)

located superolaterally on each lobe, acts on osteoclasts to inhibit calcium absorption

cuts down serum calcium

calcitonin can be stimulated by pentagastrin, synthetic calcium, alcohol