Surgical thyroid disease encompasses those conditions 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.
ANATOMY
The thyroid gland 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. The normal thyroid gland 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. The thyroid gland receives its main blood supply from the superior and inferior thyroidal arter-ies, the latter of which is shared with its neighboring parathyroid glands (Figs. 12-1 and 12-2).
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Figure 12-1 • Anatomy of the thyroid gland. |
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Figure 12-2 • Course of the recurrent laryngeal nerve. |
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, whereas 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.
The superior laryngeal nerves originate from the vagus nerves as well. It is the external branch of the superior laryngeal nerves which holds significance, given its motor innervation to the cricothyroid muscle. This latter structure controls the high pitch of voice. It lies within the superior pole vessels, and care must be taken when the superior pole of the thyroid is being removed.
PHYSIOLOGY
The purpose of the thyroid gland is to 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).
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Figure 12-3 • Regulation of thyroid gland secretion. TRH, thyroid-releasing hormone; TSH, thyroid-stimulating hormone. From Premkumar K. The Massage Connection Anatomy and Physiology. Baltimore: Lippincott Williams & Wilkins, 2004. |
GRAVE'S DISEASE AND TOXIC NODULAR GOITER
Grave's disease and toxic nodular goiter (TNG) encompass the majority of hyperthyroidism cases. TNG has also been referred to as Plummer's disease. Whereas Grave's disease is an autoimmune process in which the body's own antibodies stimulate the TSH receptor, causing excess T4 and T3 in addition to gland growth, the mechanism of action of TNG is through autoproduction of thyroid hormones regardless of TSH control.
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HISTORY AND PHYSICAL EXAMINATION
Both diseases clinically can lead to similar symptoms and signs, such as anxiety, heat sensitivity, nervousness, 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).
DIAGNOSTIC EVALUATION
Laboratory tests consist of TSH and free T4. One finds the TSH to be suppressed to near zero, with elevated 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.
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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 performed to minimize risk of hyperthyroidism recurrence. However, 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 preoperatively 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. Although thyroid nodules are common in the general population, with a higher incidence in women than in men, only a small portion are clinically evident, and within this group, nearly 90% of the nodules are benign, with the rest being malignant. However, 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 (<20 years) and old age (>60 years), rapid enlargement of the nodule, voice changes, and presence of thyroid disease. The 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. The cytologic specimen can be evaluated immediately for adequacy and morphology, determining whether the
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lesion is benign, malignant, or indeterminate. Light microscopy remains the gold standard, although molecular markers, microarrays, and associated gene mutations are coming into play. These latter tools may further differentiate the numerous classification of thyroid cancers involved in a continuum from well differentiated to poorly differentiated, allowing appropriate treatment.
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 calcifications. In addition, ultrasound evaluation of any nearby lymphadenopathy can further predict risk of malignancy. Nodules 1 to 1.5 cm are considered biologically 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 diagnosis, 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. Classically, the mechanism of action has been TSH stimulation, but now consideration is given to nodular growth that occurs with age and is made worse with other exposures, such as excess iodine, Hashimoto thyroiditis, environmental goitrogens, Grave's, and lithium.
Indications for surgery include neck compression syndromes affecting swallowing, breathing, and speaking, where malignancy cannot be excluded; hyperthyroidism and progressive growth in the presence of suppression 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
Thyroid cancer incidence is greater in women than men (3% versus 1%, respectively) for unknown reasons. Compared with other cancers with greater incidence, such as breast, lung, and prostate, thyroid cancer mortality is favorable overall. 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.
STAGING
Staging for thyroid cancer is according to the TNM classification (Table 12-1).
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TABLE 12-1 American Joint Committee on Cancer (AJCC) TNM Classification of Thyroid Carcinoma |
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TREATMENT
Papillary
Surgery is the mainstay of treatment for papillary carcinoma and its variants. There has been much discussion regarding extent of surgery, which can range from lobectomy and isthmectomy to total thyroidectomy. Papillary cancer can be multicentric, spreading into nearby lymph nodes. Lymph node dissection is performed selectively. In selected patients where the cancer is confined to one lobe and is <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
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cancers, can be performed after total thyroidectomy and at follow-up to monitor signs of recurrence.
Follicular
A follicular lesion 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 (>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. Follicular cancer 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.
Medullary
In the familial form of the disease, whether isolated or associated with multiple endocrine neoplasia 2A or 2B, medullary thyroid cancer is known to metastasize 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 thyroidectomy with central neck dissection is performed (see Color Plate 11).
Anaplastic/Lymphoma
Rarely are anaplastic thyroid cancer and lymphoma treated surgically. Both processes present with a rapidly enlarging neck mass. The undifferentiated anaplastic cancer is considered lethal with a short survival time of months, whereas the prognosis for lymphoma is better. FNA with appropriate immunohistochemical staining is used for appropriate diagnosis, but sometimes open biopsy for tissue histology is confirmatory. Non-Hodgkin B-cell type is the most common lymphoma pathology for thyroid. Airway management rather than thyroidectomy is the surgical intervention of choice.
COMPLICATIONS OF THYROID SURGERY
There are unique risks involved in thyroid surgery that increase with extent of surgery. The most important is injury to the recurrent laryngeal nerve, which not only
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controls voice but airway function. The best prevention is identification of the nerve by visualization and assistance with use of intraoperative nerve monitoring. Even if the nerve is not known to be injured, vocal cord paresis can occur postoperatively. Temporary paresis/ paralysis of the nerve can last up to 3 months. Symptoms include voice changes and aspiration of liquids. Bilateral vocal cord paralysis requires tracheostomy. Long-term paralysis may be treated with synthetic injection such as Teflon, gel foam, or collagen; this technique is known as laryngoplasty.
Injury to the external branch of the superior laryngeal nerve can occur during the takedown of the superior pole of the thyroid. The result is minimization of high pitch, which may not be apparent except when singing and yelling.
Calcium balance disruption occurs postoperatively from parathyroid manipulation and devascularization. It is not uncommon to see hypocalcemia immediately after surgery, which can be supplemented with oral as well as intravenous calcium. Vitamin D is added if calcium addition alone is insufficient. This condition is temporary. However, if the parathyroids are known to be removed or permanently devascularized and no reimplantation occurs, permanent hypoparathyroidism is likely to occur. These patients will need lifelong calcium and Vitamin D supplementation.
Postoperative bleeding in the wound bed given the location is life-threatening. The expanding hematoma causes compression on the airway, and this surgical emergency should be addressed in the operating room.
Key Points