Thyroid Flashcards
Original attachment of thyroid in oral cavity is where?
foramen cecum
Foramen cecum becomes what?
thyroglossal duct
C-cells of thyroid make what hormone?
calcitonin
Thyroid gland is endodermal in origin, what part of thyroid gland not endodermal?
C-cells
Thyroid tissue found in neck lateral to jugular vein is considered what?
thyroid Ca– typically papillary
Two pyramidal lobes are joined together at the ?
isthmus
What % of pts have a pyramidal lobe?
30% -50%
This suspensory ligament is main point of fixation of thyroid posteriorly and laterally;
ligament of Berry
Surgical importance of ligament of Berry?
close association with recurrent laryngeal nerve
Course of recurrent laryngeal nerves?
ascend on either side of the trachea
lie lateral to ligament of Berry as they ascend the larynx
On the right side, the recurrent laryngeal nerve crosses under what artery?
right subclavian
During neck dissection, how can we find the right recurrent laryngeal nerve>
1cm lateral to or within the tracheo-esophageal groove
usually at level of lower thyroid border
Most commonly encountered congenital cervical anaomaly?
thyroglossal duct cysts
What causes thyrglossal duct cysts?
by 5th week of gestation thyroglossal duct starts to obliterate, completed by 8th week
rarely it can persist in whole or in part
80% of thyroglossal duct cysts are found where?
next to hyoid bone
How do we diagnose thyroglossal duct cysts?
smooth midline neck mass that moves with swallowing
Tx for thyroglossal duct cyst?
Sistrunk procedure; en bloc cystectomy and excision of the central hyoid bone to minimize recurrence
What % of thyroglossal duct cysts are found to contain cancern?
1%
usually papillary
What cancer do we never find in thyroglossal duct cysts?
medullary
What’s a lingual thyroid?
failure of thyroid tissue to descend during development
can be the only thyroid tissue present in the body
When do we perform interventions for lingual thyroid?
when there is concern for choking, dysphagia, airway compromise, hemorrhage
Before treating someone for lingual thyroid, what needs to be done first?
need evaluate for normal thyroid tissue in the neck or else the pt can become hypothyroid once lingual thyroid removed
Medical tx for lingual thyroid?
exogenous thyroid hormone administration to suppress TSH
administration of radio-active iodine followed by hormone replacement
What is ectopic thyroid?
normal thyroid tissue can be found anywhere along the central neck region
includes esophagus, trachea, anterior mediastinum
If we see thyroid tissue lateral to the carotid sheath and jugular vein, previously termed lateral aberrant thyroid tissue, we should be concerned about what?
usually its metastatic thyroid cancer (papillary mostly)
Normal thyroid gland weighs how much?
20g
What % of pt’s have a pyramidal lobe?
30-50%
The thyroid capsule is condensed into the posterior suspensory ligament called:
Berry’s
Superior thyroid arteries arise from?
ipsilateral external carotid A
superior thyroid A divides into anterior and posterior branches at thyroid apex
Inferior thyroid arteries arise from what?
thyrocervical trunks from ipsilateral subclavian arteries
Superior and inferior thyroid arteries originate from where?
STA–> external carotid A
ITA–> thyrocervical trunk from subclavian A
Whats the thyroid IMA artery?
seen in 1-4 % of pts
arises directly from aorta or innominate and enters isthmus (sometimes replacing an inferior thyroid artery)
When dissecting the inferior thyroid artery and before ligating the inferior thyroid artery, what needs to happen?
need to identify the recurrent laryngeal nerve
ITA crosses the recurrent laryngeal nerve
Venous drainage of the thyroid?
superior V–> internal jugular
middle V–> internal jugular
inferior V–> form a plexus–> drain into brachiocephalic veins
Describe the course of the left recurrent laryngeal nerve:
arises from vagus
crosses aortic arch
loops around ligamentum arteriosum
ascends medially in neck along tracheo-esophageal groove
Describe the course of the right recurrent laryngeal nerve;
arises from right vagus
crosses under right subclavian artery
enter larynx posterior to cricothyroid m.
What m. do the recurrent laryngeal nerves innervate?
all intrinsic m. of larynx
except cricothyroid m
What nerve innervates the circothyroid m.?
external laryngeal nerve
What happens when we have injury to one recurrent laryngeal nerve?
ipsilateral vocal cord paralysis
cord stuck in paramedia/abducted position
**pts have hoarse voice, ineffective cough
B/l recurrent laryngeal nerve injury causes what?
airway obstruction necessitating trach
The superior laryngeal nerve branches off into what?
internal
external
What does internal branch of superior laryngeal nerve innervate?
sensation to supraglottic larynx
injury to this nerve is rare, but injury can lead to aspiration
What does the external branch of superior laryngeal nerve innervate?
circothyroid m.
Injury to the external branch of superior laryngeal nerve causes what?
difficulty with pitch, can’t hit high notes
Most common anatomic variant of external branch of superior laryngeal nerve as it enters circothyroid m?
type 2A variant
nerve crosses over below tip of superior pole of thyroid
seen in 20% of pts
puts nerve at greatest risk of injury
Damage to a recurrent laryngeal nerve leads to?
ipsilateral vocal cord paralysis
When dissecting the superior thyroid artery, care must be taken not to damage what nerve?
external branch of superior laryngeal nerve
Describe path of inferior thyroid artery?
comes off of thyrocervical trunk from subclavian A
enters thyroid posteriorly near ligament of Berry
Thyroid IMA artery comes off brachiocephalic or aorta, and seen in what % of pts?
5-10 %
When dissecting the inferior thyroid artery, care must be taken not to damage what nerve?
recurrent laryngeal
Inferior thyroid A supplies which parathyroids?
inferior + superior
Most thyroid cancers drain into what lymph node region?
central lymph nodes (VI)
How do we locate the parathyroids?
follow branches of inferior thyroid A into the parathyroids
Blood supply of the parathyroids?
inferior thyroid artery
if inferior thyroid artery is sacrificed on that side of the gland, parathyroids have no collaterals
What hormones does thyroid make?
T4; thyroxine
T3; tri-iodo-thyronine
Calcitonin
Describe iodine metabolism:
we get iodine from fish, eggs, salt,
converted to iodide in stomach/jejunum
absorbed in bloodstream
iodide then actively transported into thyroid follicles
Which is more potent, T3 vs T4?
T3 is more potent
T3 plasma circulating levels less than T4
T3 is less bound to plasma proteins, enters tissues more rapidly
Half lives of T3 and t4?
T3 is 1 day
T4 is 7 days
What controls thyroid secretion?
hypothalamic-pituitary-thyroid axis
hypothalamus release TRH–> stimulates pituitary to make TSH
TSH then mediates iodide trapping, release of thyroid hormones
TRH and TSH can be negatively fed back by T3 and T4
What enzyme is important in thyroid hormone synthesis ?
thyroid peroxidase
helps form MIT, DIT
Most thyroid hormone released from thyroid gland is in the form of?
T4
T4 gets de-iodinated in peripheral tissues to T3
What hormone facilitates conversion of MIT and DIT into T3 and T3?
thyroglobulin
Which cells make calcitonin?
parafollicular C-cells
Where do we find parafollicular C-cells which make calcitonin?
supero-laterally in each thyroid
What does calcitonin do?
inhibits calcium absorption by osteoclasts
What hormones stimulate thyroid hormone production?
epinephrine
HcG
What hormones inhibit thyroid hormone production?
steroids
What do thyroid hormones do?
vital in brain development and skeletal maturity
increase O2 consumption, basal metabolic rate, heat production
+ inotropic/chronotropic effects
affect GI motility
increase bone and protein turnover
Which thyroid hormone responsible for negative feedback loop on TSH in pituitary and TRH in hypothalamus?
T3
Specific actions of calcitonin take effect on surface of what cells?
osteoclasts
Two anti-thyroid drugs?
PTU (propylthiouracil)
methimazole
MOA of PTU and methimazole?
inhibit organification and oxidation of inorganic iodine
inhibit linking of MIT and DIT
**PTU inhibits peripheral conversion of T4 to T3
WHich anti-thyroid drug can we use in pregnant pts?
PTU
Which anti-thyroid drug preferred in non-pregnant pts?
methimazole
Major, but rare, side effect of PTU and methimazole?
agranulocytosis
Side effects of PTU and methimazole?
agranulocytosis ( less than 1 %)
rash
arthralgias
liver dysfunction
How do steroids affect thyroid hormones?
exogenous steroids suppress the pituitary-thyroid axis
prevent conversion of T4 –>T3 in periphery
**this allows steroids be used as inhibitory agent in hyperthyroid conditions
What happens when we have iodine excess?
iodine transport and synthesis and secretion of thyroid hormones are inhibited
excessive large doses initially lead to increased organification followed by suppression ; Wolff-Chaikoff effect
Wolf-Chaikoff effect?
high doses of iodine initially lead to increased organification followed by suppression of thyroid hormones
Iodine when given in large doses, can inhibit thyroid hormone release by altering the organic binding process, what is this called?
Wolf-Chaikoff effect
In terms of thyroid testing, what test is most sensitive and specific for diagnosis of hyper-hypo thyroid states?
TSH assay
Why are T4 levels not reliable screening tests for thyroid dx?
T4 levels not only increased in pts with hyperthyroid states but also in pts w/;
elevated thyroglobulin levels such as; pregnancy, estrogen/progesterone use
decreased in anabolic steroid use; nephrotic syndrome
Accurate evaluation of thyroid function involves measuring free T4 and T3, why?
total T4 and T3 assays measure free and protein bound hormone, which can be affected by changes in hormone production and hormone binding
About 80% of pts with Hashimoto’s thyroiditis have elevated thyroid antibody levels, what antibodies?
anti-thyroglobulin
anti-microsomal
anti-TPO
What use does thyroglobulin have in measuring levels?
helpful in monitoring pts with differentiated thyroid cancer for recurrence particularly after thyroidectomy and RAI
What radio-active iodine isotopes do we use to image thyroid?
I- 123 (low dose radiation, used to evaluate lingual thyroids and goiters)
I-131 (higher dose radiation; used to evaluate pts with differentiated thyroid cancers for mets)
What’s a cold vs hot thyroid nodule?
cold nodule–> trap less radioactivity than the surrounding gland
hot nodule–> areas that demonstrate increased activity compared to surrounding gland tissue
The risk of thyroid malignancy is highest with cold vs hot nodules?
cold nodules have a higher risk; 20%
hot nodules have less risk; <5%
Deficiency of circulating levels of thyroid hormones lead to hypothyroidism, which is termed what in neonates?
cretinism
Causes of hypothyroidism in developing vs developed countries?
developing; iodine deficiency
developed; Hashimoto’s thyroiditis, radioactive iodine therapy, or surgical removal
What are some clinical features of hypothyroidism?
neonates; severe mental retardation, failure to thrive
adults; tiredness, weight gain, cold intolerance, constipation, menorrhagia, dry skin, brittle hair, loss of outer 2/3 of eyebrows, low libido, bradycardia
Pts w/severe hypothyroidism tend to develop myxedema, facial puffyness, periorbital puffyness due to?
deposition of glycosaminoglycans
What drugs can cause hypothyroidism sometimes?
too much PTU, methimazole
amiodarone
lithium
In primary hypothyroidism, what lab values can we see?
low circulating levels of T3 and T4
elevated TSH levels
Causes of secondary hypothyroidism?
pituitary tumor
pituitary resection or ablation
** we see decreased TSH levels
Tx for hypothyroidism?
levothyroxine; T4
available PO, IM, IV
Major cause of hypothyroidism is?
Hashimoto’s thyroiditis; auto-immune mediated destruction of thyrocytes
What antibodies do we see in Hashimoto’s thyroiditis which causes primary hypothyroidism?
anti-thyroglobulin
anti-thyroperoxidase
anti-TSH-R
This is an autoimmune process which is initiated by activation of CD4 cells with specificity for thyroid antigens;
Hashimoto’s thyroiditis
CD4 T cells recruit cytotoxic CD8 cells which destroy thyrocytes, in combination with auto-antibodies
What do we see on microscopy of Hashimoto’s thyroiditis?
thyroid tissue infiltrated by small lymphocytes and plasma cells
Hashimot’s is more common men or women?
women
How does Hashimoto’s present clinically?
minimally enlarged firm granular gland discovered on routine PE
painless anterior neck mass
How do we confirm diagnosis of Hashimoto’s?
elevated TSH levels + presence of thyroid Ab
Serious complication of Hashimoto’s thyroiditis?
lymphoma
**has 80 x higher prevalence than in general population
Tx for Hashimoto’s?
thyroid hormone replacement therapy
surgery if suspicious for malignancy or goiters that cause compressive sxs
What is Riedel’s thyroiditis?
rare entity
replacement of all part of thyroid by fibrous tissue
tissue also invades nearby structures, trachea, esophagu
How does Riedel’s thyroidits present?
seen in women 30-60
painless, hard, anterior neck mass
see symptoms of compression; dysphagia, choking, hoarseness
pts have sx of hypothyroidism as gland is replaced by fibrous tissue
you feel a hard and ‘woody’ thyroid
Hard ‘woody’ thyroid on exam, makes you think of?
Riedel’s thyroiditis
How do we diagnose Riedel’s thyroiditis?
open biopsy
the firm and fibrous nature of the gland makes FNAB difficult
Mainstay of tx for Riedel’s thyroiditis?
surgery; decompress the trachea by wedge resection of thyroid isthmus
hypothyroid pts; tx w/thyroid replacement
some pts benefit from steroids and tamoxifen
Microscopically what does RIedel’s thyroiditis look like?
dense fibrous tissue
almost total obliteration of normal follicular structure
What is acute suppurative thyroiditis?
thyroid gland is usually resistant to infection due to rich blood supply, lymphatics, fibrous capsule
infectious agents can still seed it via hematogenous/lymphatic spread
MOst common bacteria causing acute suppurative thyroiditis?
streptococcus species + anaerobes make up 70%
**more common kids, preceded by URI/OM
Tx for acute suppurative thyroiditis?
antibiotics
drainage of any abscesses
Causes of subacute thyroiditis?
autoimmune vs post-viral in origin
affects women 2;1
Tx for subacute thyroiditis?
usually NSAIDs + steroids effective
