The word thyroid is derived from the Greek thyreos, or shield. The thyroid gland originates from two sites. The tuberculum impar at the base of the vallate papillae of the tongue forms the foramen cecum. The thyroid tissue descends into the neck along the line of the thyroglossal duct. The lateral component, which contributes the calcitonin-producing C cells, is derived from the fourth pharyngeal pouch.

The mature thyroid gland, which weighs about 20 g, drapes over the anterolateral aspect of the upper trachea just below the cricoid cartilage. The two lobes, connected by the isthmus, lie along the sides of the larynx and trachea, extending up to the level of the middle of the thyroid cartilage. The pyramidal lobe is a diverticulum extending upward from the isthmus. These lobes are between the trachea medially and the carotid sheaths and sternocleidomastoid muscles laterally; the strap muscles lie anterior to the thyroid lobes. The parathyroid glands and the recurrent laryngeal nerves are on the posterior surface of the lateral lobes of the thyroid gland. The nerves are typically found in the tracheoesophageal groove. A thin, fibrous capsule surrounds the thyroid gland.

Surgical treatment of thyroid disease should be considered for the following conditions:

Four main types of thyroid cancers can be distinguished according to their pathology: papillary, follicular, medullary, and anaplastic. More than 90% of all thyroid cancers are papillary and follicular (well-differentiated) carcinomas.

Papillary carcinomas are well-differentiated lesions. The peak incidence is in the third to fourth decades, and women are affected three times as often as men. Papillary carcinomas contain concentric layers of calcium called psammoma bodies. Papillary cancers are the slowest-growing thyroid cancers, and they spread through the lymphatic system. When thyroid tissue is found in the posterior triangle of the neck, the condition has been referred to as lateral aberrant thyroid. This actually represents a papillary cancer that has metastasized to a lymph node. Accessory thyroid tissue, separate from the thyroid, can be found in the central compartment of the neck. Usually this does not represent metastatic thyroid cancer but rather an embryonic rest of thyroid tissue. The multicentricity rate of papillary thyroid cancer is 80% on microscopic examination, but probably less than 5% have a clinical effect. Papillary metastases usually concentrate iodine and can be treated with radioiodine. Some papillary thyroid cancers show features of follicular cancer and are known as follicular variants of papillary cancer. They behave in the same way as papillary cancers.

Follicular carcinomas are generally also well-differentiated tumors. Pure follicular cancers, as distinct from follicular variants of papillary cancer, differ from papillary tumors in that they exhibit less multicentricity and are more likely to spread hematogenously to bone, lung, and liver. Follicular metastases may concentrate iodine, but to a lesser degree than papillary lesions. When iodine uptake is demonstrated, they can be treated with radioiodine.

Medullary carcinomas are less common tumors, comprising 3% to 5% of thyroid cancers. They are calcitonin-producing tumors that originate from the C cells; amyloid deposits in the stroma of the tumor are diagnostic. Bilateral multicentricity is more commonly found in familial cases but is still common in the sporadic variety. Medullary cancers occur in families as components of both multiple endocrine neoplasia (MEN) type II and the familial medullary carcinoma syndrome (see Chapter 23). MEN type IIA, or Sipple’s syndrome, also includes pheochromocytomas and hyperparathyroidism (HPT). MEN type IIB is a less common form that includes mucosal neuromas, ganglioneuroma of the bowel, and a marfanoid appearance; HPT is rare. An elevated serum calcitonin level is diagnostic of medullary carcinoma and is also a useful screening tool for familial transmission and to monitor recurrence. Evaluation of point mutations in the Ret protooncogene is part of the workup of these patients and their families (2).

Anaplastic carcinomas account for less than 5% of thyroid cancers and occur later in life than the other types. These cancers are characterized by extremely rapid and widespread growth. Patients present with a sometimes painful and enlarged thyroid gland. These tumors are highly aggressive and are associated with a very short life expectancy.

The lesion must be excised, but the extent of resection—thyroid lobectomy alone versus total thyroidectomy (Fig. 4.2)—has been debated extensively (3,4). Lobectomy is advocated by some because it minimizes the operative risks of nerve and parathyroid injury; multicentric disease rarely becomes a clinical problem; and thyroid cancers in general are not very aggressive. More extensive surgical ablation (total thyroidectomy) is promoted because it allows postoperative scanning with ¹³¹I for metastatic disease because no normal thyroid tissue (which concentrates iodine more vigorously than papillary or follicular metastases) remains in the neck; it facilitates follow-up with thyroglobulin levels; and it eliminates recurrence caused by multicentricity. One large study has shown a survival benefit for total thyroidectomy over lobectomy (5). A majority of endocrine surgeons favor total thyroidectomy for lesions larger than 2 to 3 cm.

After total thyroidectomy, L-thyroxine is given for life in doses sufficient to maintain normal levels of TSH. Radioiodine is used to ablate any residual functional thyroid tissue in the neck after thyroidectomy and to destroy widespread metastases. Improved survival was demonstrated after treatment with total thyroidectomy, radioiodine ablation, and TSH suppression for life in a study of 1355 patients who were followed for 40 years (5,6).

The parathyroids are very delicate and despite meticulous dissection, clinically significant hypoparathyroidism after thyroid resection is usually a temporary problem. The signs and symptoms of hypoparathyroidism occur within the first few days after surgery. The early symptoms are perioral numbness, tingling of the fingers, and in extreme cases, carpopedal spasm. The threshold of muscular excitability is lowered, and involuntary spasms occur. The following are classic signs of hypo-parathyroidism:

The calcium level is measured postoperatively in any patient undergoing total thyroidectomy. It is probably unnecessary to perform this study when only one side has been explored. If the patient is symptomatic or if the calcium level falls to less than 7.5 to 8 mg/dL, oral calcium replacements are given (500 mg of elemental calcium in the form of 1250 mg of calcium carbonate (Os-Cal) or 600 mg. of calcium as 1500 mg. of calcium citrate (Caltrate) every 6 hours). Most postoperative hypoparathyroidism resolves over the first few days or weeks. If the hypocalcemia is unresponsive to oral calcium supplementation, oral vitamin D (calcitriol 0.25 to 0.5 µg daily) may be given.

Several prognostic scoring systems have been developed for well-differentiated thyroid cancer. Two such systems are the AGES system (age of patient; grade, extent, and size of tumor) (11) and the AMES system (age, metastatic disease, and extent and size of tumor) (12). Papillary carcinoma has the most favorable prognosis, with approximately 95% survival at 10 and 20 years. The prognosis for follicular and medullary carcinomas is less favorable, with 80% of patients surviving at 10 years and 71% at 20 years (13). Anaplastic carcinomas have a poor prognosis: fewer than 10% of patients survive 1 year (14).

The follow-up for well-differentiated carcinomas consists of physical examination, thyroglobulin measurements, ultrasonography, chest radiography, and ¹³¹I nuclear scanning. For patients with medullary carcinoma, the follow-up is similar, but it also includes calcitonin measurement and molecular analysis of the Ret protooncogene. This is important for family members for evaluation of familial carcinoma.

Goiter is benign enlargement of the thyroid gland. The term is derived from the Latin guttur, which means throat. The pathogenesis of multinodular goiter in most cases is unknown; the involvement of growth factors is being investigated (15). TSH clearly plays a role in iodine deficiency states, but iodine deficiency is rare in the United States today.