Endocrine: Thyroid Gland Flashcards
Thyroid Gland
- in which partial or complete removal of the thyroid gland is required as a result of goiter and hyperthyroid conditions that are unresponsive to medical management or benign and malignant neoplastic disease
Thyroid Gland: Anatomy
- develops embryologically from the first and second pharyngeal pouches, migrating caudally to form a butterfly-like structure anterior to the trachea at the level of the second tracheal ring
- The thyroglossal duct is the remnant of the tract.
- Neural crest cells which are the source of the C cells that produce calcitonin, are also involved in the process of thyroid formation.
- is bilobed, weighing 15 to 25 g
- there is usually an isthmus connecting the two lobes, as well as a pyramidal lobe, which is superior to the isthmus.
- its main blood supply from the superior and inferior thyroidal arteries, the latter of which is shared with its neighboring parathyroid glands (Figs. 12-1 and 12-2)
- In close proximity to the thyroid are the paired recurrent laryngeal nerves (RLNs), which control the cricopharyngeus muscle as well as the vocal cords.
- The RLNs originate from the vagus nerves in the chest.
- The right RLN runs behind the subclavian artery at the base of the neck more lateral than the left RLN at this level. Occasionally the right RLN may be nonrecurrent.
- The right RLN tends to course lateral to medial as it approaches the inferior thyroidal artery from the base of the neck
- the left RLN courses with less angulation, running parallel to the tracheoesophageal groove.
- The RLN enters the larynx at the level of the inferior constrictor muscles.
- superior laryngeal nerves
- originate from the vagus nerves as well
- is the external branch of the superior laryngeal nerves which holds significance,
- given its motor innervation to the cricothyroid muscle.
- controls the high pitch of voice
- lies within the superior pole vessels, and care must be taken when the superior pole of the thyroid is being removed.
Thyroid Gland: Physiology
- maintain the body’s metabolism via the hormone thyroxine (T4) or triiodothyronine (T3), produced by the follicular cells.
- A negative feedback loop with the hypothalamus and pituitary controls the state of the gland via thyroid-stimulating hormone (TSH; Fig. 12-3).
Grave’s Disease and Toxic Nodular Goiter (TNG)
- encompass the majority of hyperthyroidism cases
- referred to as Plummer’s disease.
- an autoimmune process in which the body’s own antibodies stimulate the TSH receptor, causing excess T4 and T3 in addition to gland growth
- mechanism of action of TNG is through autoproduction
of thyroid hormones regardless of TSH control
Grave’s Disease and Toxic Nodular Goiter (TNG): History and Physical Examination
- Both diseases clinically can lead to similar symptoms
and signs, such as anxiety, heat sensitivity, nervous-
ness, weight loss, fatigue, palpitations, tachycardia,
and palpable goiter, but findings unique to Grave’s
disease are eye proptosis, which is irreversible, and
pretibial myxedema (see Color Plate 9).
Grave’s Disease and Toxic Nodular Goiter (TNG): Diagnostic Evaluation
- Laboratory tests consist of TSH and free T4. One
finds the TSH to be suppressed to near zero, with ele-
vated free T4. Radioactive iodine uptake testing will
differentiate thyroiditis from Grave’s disease but is
not necessary for confirmation if the clinical picture
is consistent with Grave’s disease. In TNG, diffuse
areas of uptake will be seen.
Grave’s Disease and Toxic Nodular Goiter (TNG): Treatment
- Hyperthyroidism treatment can be medical, with use
of radioactive iodine or, in the case of Grave’s disease,
drugs that inhibit synthesis of thyroid hormones,
such as methimazole and propylthiouracil.
- In addition, beta-blockers can be used for symptomatic relief in Grave’s disease as well.
- Surgery is the best option when patients are symptomatic from their goiters (e.g., dysphagia or pressure-like sensations in the neck), when malignancy cannot be excluded in the enlarged thyroid, and when radioactive iodine is not a suitable option (children, pregnant women, resistant cases, and patients not agreeable to risks of radioactive iodine).
- Specifically, bilateral subtotal thyroidectomy is per-
formed to minimize risk of hyperthyroidism recur-
rence.
* surgery also puts the patient at risk
for long-term hypothyroidism, particularly in Grave’s
disease, where a euthyroid state rarely is achieved
after surgery alone.
* All patients preoperatively must be made euthyroid using methimazole or propylthiouracil. Iodide preparations are avoided in patients preoperatively as they may worsen the patient’s condition, whereas iodides are often used preopera tively in Grave’s disease to minimize vascularity of the thyroid
Thyroid Nodule
- Evaluation of the thyroid nodule is what leads a patient eventually to surgery.
- a higher incidence in women than in men
- small portion are clinically evident, and within this group, nearly 90% of the nodules are benign, with the rest being malignant.
- risk factors that would lead one to suspect malignancy include history of head/neck radiation, family history of thyroid cancer (familial medullary thyroid carcinoma, multiple endocrine neoplasia syndrome), male sex (because more female patients present with benign thyroid nodules than male patients), very young age (less than 20 years) and old age ( greater than 60 years), rapid enlargement of the nodule, voice changes, and presence of thyroid disease.
- palpable nodule on examination or, as is now being seen more often, incidental nodules on radiologic studies for other purposes, can be definitively tested with fine-needle aspiration (FNA).
- The technique of FNA involves sampling the contents of the nodule in several planes via a 25-gauge needle on a syringe.
- can be evaluated immediately for adequacy and morphology, determining whether the lesion is benign, malignant, or indeterminate.
- Light microscopy remains the gold standard, although molecular markers, microarrays, and associated gene mutations are coming into play.
- latter tools may further differentiate the numerous classification of thyroid cancers involved in a continuum from well differentiated to poorly differentiated, allowing appropriate treatment
Thyroid Nodule: Algorithm
- After taking an adequate history and conducting a
physical examination, the patient’s TSH level should
be checked, after which point an ultrasound should
be performed.
- The ultrasound can characterize the
nodule by echogenicity, borders, vascularity, and cal-
cifications. In addition, ultrasound evaluation of any
nearby lymphadenopathy can further predict risk of
malignancy.
- Nodules 1 to 1.5 cm are considered bio-
logically significant and should undergo FNA.
- Repeat FNA is recommended in the event of an
inadequate specimen.
- Presence of follicular or Hürthle
cells without malignancy is an indication for surgical
intervention, because cytological examination alone is
insufficient for determining presence of malignancy.
- Presence of capsular invasion or angioinvasion can only
be seen on histological examination where the thyroid
architecture is intact.
- In addition, frozen section of the
thyroid specimen will not necessarily confirm the diag-
nosis, as multiple sections must be investigated.
- Thus in many instances, the diagnosis will be deferred to review of permanent sections on pathological examination.
Goiters
- In most parts of the world, enlarged thyroid gland, or
goiter, is due to iodine deficiency; however, in the United
States, it is most likely due to Hashimoto thyroiditis.
- the mechanism of action has been TSH stim-
ulation, but now consideration is given to nodular
growth that occurs with age and is made worse with
other exposures, such as excess iodine, Hashimoto thy-
roiditis, environmental goitrogens, Grave’s, and lithium.
- Indications for surgery include neck compression
syndromes affecting swallowing, breathing, and speak-
ing, where malignancy cannot be excluded; hyperthy-
roidism and progressive growth in the presence of sup-
pression medically; and cosmesis (see Color Plate 10).
- Rather than classify goiters by size or weight criteria,
the World Health Organization in 1960 developed
a grading system for clinical evaluation, with grade 0
representing no enlargement and grade 3 representing
enlargement that is evident from a distance (Color
Plate 10).
- The majority of goiters remain in the neck;
however, a minority have substernal extension into the
anterior mediastinum.
Thyroid Cancer
- incidence is greater in women than men
(3% versus 1%, respectively) for unknown reasons.
- Multiple types of thyroid tumors exist, which can be distinguished by cell origin.
- Most common are the follicular cell neoplasms, which
include papillary and its variants; follicular and Hürthle
cell; poorly differentiated; and anaplastic. These occur
on a spectrum ranging from well differentiated to
poorly differentiated, determining biologic behavior
and subsequent treatment.
- Other cancers include medullary cancer derived from C cells and lymphoma.
- Papillary and follicular cancers tend to have a more
favorable prognosis than the medullary and anaplastic
variants
Thyroid Cancer: Staging
- Staging for thyroid cancer is according to the TNM
classification (Table 12-1).
Thyroid Cancer
Type: Papillary: Treatment
- Surgery is the mainstay of treatment for papillary carci-
noma and its variants. There has been much discussion
regarding extent of surgery, which can range from lobec-
tomy and isthmectomy to total thyroidectomy.
- Papillary cancer can be multicentric, spreading into nearby lymphnodes.
- Lymph node dissection is performed selectively.
- In selected patients where the cancer is confined to one lobe
and is less than 1.5 cm without extracapsular involvement, minimal lobectomy with isthmectomy may be sufficient.
However, total thyroidectomy is advocated to minimize
risk of local recurrence if injury to the parathyroids and
recurrent laryngeal nerves are minimal.
- Postoperatively, TSH is suppressed with thyroidal hormone replacement, and radioactive iodine may be used to ablate any residual thyroidal tissue.
- Measurement of serum thyroglobulin, which is a tumor marker for well-differentiated thyroid cancers, can be performed after total thyroidectomy and at follow-up to monitor signs of recurrence.
Thyroid Cancer
Type: Follicular: Treatment
- on FNA is not definitive in distinguishing benign from malignant, because the components of the capsule and blood vessels must be evaluated via histological examination.
- The minimal approach is to perform lobectomy and await permanent pathology reading.
- Obtaining frozen section of a follicular lesion
usually is not meaningful or cost effective.
- However, if one highly suspects follicular carcinoma based on size (greater than 4 cm), selective frozen section may be helpful or one may proceed with total thyroidectomy.
- Also, evaluation of the contralateral lobe at the time of surgery is important in deciding extent of surgery.
- After lobectomy with pathology showing minimally invasive follicular carcinoma, completion thyroidectomy is not necessarily indicated in low-risk patients.
- does not metastasize to lymph nodes as much as papillary cancer.
- Thus elective nodal dissection is not performed in the absence of adenopathy.
- In contrast to papillary cancer, follicular cancer tends to metastasize via the bloodstream. Radioactive iodine may be used in patients with unresectable, gross disease.
Thyroid Cancer
Type: Medullary: Treatment
- In the familial form of the disease, whether isolated
or associated with multiple endocrine neoplasia 2A
or 2B, medullary thyroid cancer is known to metast
asize early to surrounding lymph nodes. Patients should be
genetically tested given their risk profile and undergo
total thyroidectomy with central neck dissection at
an early age. Nearly 70% of medullary cancers are
sporadic. These patients may present with a thyroid
nodule associated with cervical adenopathy. Staining
of FNA for calcitonin and absence of thyroglobulin is
diagnostic. As with the familial forms, total thy-
roidectomy with central neck dissection is performed
(see Color Plate 11).
