Anatomy/Physiology Thyroid Flashcards
Thyroid originates from:
a) endoderm
b) mesoderm
c) ectoderm
A) endoderm
Describe developmental embryology of thyroid gland.
- 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
What is the thyroglossal duct?
embryological anatomical structure forming open connection between initial area of thyroid gland and final position
How do you get a lingual thyroid gland?
What is treatment for this?
failure of median thyroid component to descend from foramen cecum
Treatment: radioactive iodine ablation
How does thyroglossal duct cyst form?
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
What is treatment for thyroglossal duct cyst?
Sistrunk procedure
-excise entire thyroglossal duct tract and a chunk of the hyoid bone
Any malignant potential with thyroglossal duct?
epithelial lining of cyst contains some thyroid cells and some carcinomas (85% papillary type, also Hurthle, follicular, but never medullary)
What is an ectopic thyroid? What are some common locations?
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.
What is thyrothymic tract rest?
Tissue that arises in thyrothymic tract below thyroid lobes
What is Ligament of Berry? Significance?
lateral thyrohyoid ligament
internal branch of superior laryngeal nerve lies lateral to it
Arterial supply of thyroid gland?
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
Venous drainage of thyroid gland?
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
Branches off right subclavian artery?
VIT (vitamin)
V- vertebral
I- internal mammary
T- thyrocervical trunk
Branches off thyrocervical trunk?
A-SIT
A- ascending cervical
S- suprascapular
I- inferior thyroid
T- transverse cervical (present in only 30% of cases)
Where do the RLN and SLN arise?
off vagus
What is the course of the vagus nerve?
arises from medulla in brainstem and exits skull via jugular foramen
-sits in carotid sheath between internal jugular vein and internal carotid artery
Where do the right and left recurrent laryngeal nerves swing around?
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.”
What is a non-recurrent laryngeal nerve?
What % of the population has this?
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
What are the functions of the RLN?
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
What is the tubercle of Zuckerkandl?
posterior extension of lateral lobe of thyroid gland
- covers RLN
- tubercle often called posterior horn
- RLN often found lateral to tubercle
What is the superior laryngeal nerve?
-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
Where do most thyroid cancers drain?
Level VI (central compartment)
What are skip metastases in thyroid gland?
most thyroid cancers drain directly to Level VI (central compartment) except for cancers in superior third of gland, which drain to lateral compartment
Lymph node compartments?
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
What is the relationship of the parathyroid glands to the recurrent laryngeal nerve?
AI (inferior parathyroids- anterior to RLN), artificial intelligence
PS I love you: (Superior parathyroids- posterolateral to RLN)
What are the common locations of ectopic parathyroid glands?
Thyroid parenchyma- 33%
Thyrothymic ligament- 19%
Thymus- 15%
Carotid artery sheath- 7%
Mediastinum- 7%
What are the strap muscles?
TOSS
T- thyrohyoid
O- omohyoid
S- sternothyroid (posterior layer)
S- sternohyoid (anterior layer)
Describe basic thyroid unit
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
How does T4 and T3 come about?
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
What are the half lives of T3 and T4?
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
What is thyroglobulin?
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
Whtat is calcitonin?
What stimulates it?
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
