The frequency with which large goiters are encountered in the United States has progressively declined over the past several decades. This decrease is largely due to an increase in dietary iodine, particularly iodized salt. Despite the declining frequency of goiter diagnoses in the United States, substernal goiter remains a significant consideration in the differential diagnosis of mediastinal masses, particularly those located in the anterior mediastinum. Substernal goiter is generally defined as a thyroid mass that has 50% or more of its volume located below the thoracic inlet.
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Substernal goiters can remain asymptomatic for many years. Consequently, incidental discovery of these goiters is not uncommon, for example, on routine chest radiography. When symptoms arise, they are usually related to compression and compromise of adjacent structures from continued growth of the goiter within the bony confines of the thoracic cavity.  Symptoms of substernal goiter typically develop slowly and insidiously over long periods. Because such goiters can remain hidden within the thoracic cavity, they can occasionally become massive before discovery.
Conversely, while symptoms of substernal goiter can often develop slowly over many years, serious symptoms, such as airway compromise, can develop precipitously with little or no advance warning. Malignancy usually must be ruled out when substernal goiters are discovered in asymptomatic individuals, although fine-needle aspiration is often difficult or inadvisable in this location. For these reasons, many authors have advocated surgical removal of all substernal goiters, even when these goiters are asymptomatic.
The incidence of substernal goiter is difficult to assess but reportedly ranges from 2.6-21% of patients undergoing thyroidectomy. The wide range in reported incidence is largely due to variation in the definition of substernal goiter. Substernal goiters represent up to 7% of mediastinal tumors. 
Most (85-95%) substernal thyroid masses represent benign goiter. Historically, goiters have generally occurred because of iodine deficiency, although this is now observed primarily in developing nations. Goiters can also be caused by ingestion of goitrogens (ie, substances that block formation of thyroid hormone), which can be found in turnip, cabbage, and kale, or they can occur as a familial form.
Malignancy occurs in 5-15% of substernal goiters.
Substernal goiters are largely considered to result from the descent of a cervical goiter with the primary blood supply remaining in the neck, primarily from the inferior thyroid artery. Some theories postulate that small cervical thyroid nodules descend beneath the pretracheal fascia, possibly aided by the negative intrathoracic pressures that normally occur during respiration and swallowing. As the nodule gradually enlarges, it eventually becomes trapped below the thoracic inlet, where it continues to enlarge within the confines of the thoracic cavity, commonly hidden from view.
Substernal goiters originating from truly ectopic thyroid tissue located in the mediastinum are controversial. Arguments have been advanced that goiters, which are isolated within the mediastinum, may actually represent extensions of cervical goiters that have become sequestered (remaining connected only by fascial attachments), rather than arising truly de novo in the chest.
As substernal goiters enlarge within the bony thoracic cavity, vascular and visceral structures may slowly become compressed and compromised. Goiters often enlarge posteromedially to the great vessels and have the potential of compressing the vascular structures against the bone of the sternum or thoracic cage. Such compression can lead to superior vena cava syndrome.  Airway obstruction can occur because of compression or displacement of the trachea. Furthermore, over long periods, such compression can result in tracheomalacia. Dysphagia may originate because of compression of the esophagus. Dysphonia can occur because of compressive effects on the trachea or compromise of the recurrent laryngeal nerve.
Substernal goiters may be discovered incidentally in asymptomatic patients. For example, routine chest radiography may reveal a mediastinal mass or tracheal deviation.
The most common symptoms of substernal goiter result from compression of the trachea and/or esophagus and include dyspnea, choking sensation, cough, and dysphagia. Progressive hoarseness and superior vena cava syndrome are less common symptoms.
Hyperthyroidism may be observed in cervical or substernal goiters. Thyrotoxicity in goiters may be due to an autonomously functioning nodule or may be precipitated by ingestion of iodides found in certain expectorants or in radiographic contrast media.
Most authors consider the mere presence of a substernal goiter an indication for surgical removal. The rationale for this recommendation is primarily related to the risk of serious complications that can occur because of compression of visceral or vascular structures within the bony thoracic cavity, the most significant of which is airway compromise. Recognize that airway compromise, while generally occurring slowly and progressively, can occur precipitously, thus resulting in the need for emergent airway management.
Additional arguments in favor of thyroidectomy for even small substernal goiters include the relative ineffectiveness of suppressive therapies and concerns that delaying surgery until symptoms develop may render the operation more technically difficult.
Finally, malignancy has been reported in 5-15% of substernal goiters, and fine-needle aspiration for cytology may be dangerous or impossible. However, reasonable arguments could be made that close observation may be appropriate for a small substernal goiter observed in an asymptomatic patient of very advanced age without radiographic evidence of visceral or vascular compromise.
Substernal goiters most commonly descend into the anterosuperior mediastinum. Recognize that the great vessels may be near the capsule of the gland or displaced anteriorly or inferiorly, although the predominant vascular supply is cervical. Typically, substernal goiters descend to one side of the trachea or the other, causing tracheoesophageal deviation to the contralateral side. Centrally located substernal goiters may compress the trachea between the sternum and spine, usually displacing the esophagus. When goiters are located in the anterosuperior mediastinum, the recurrent laryngeal nerve is not typically displaced from the tracheoesophageal groove.
Occasionally, substernal goiters may extend into the posterior mediastinum behind the trachea or even into the posterior thoracic cavity. These goiters are typically associated with greater anatomic considerations and complexity. Such goiters may extend between the trachea and esophagus, significantly displacing these structures or greatly distorting the normal relationship of the recurrent laryngeal nerve with the trachea. Goiters located in the posterior mediastinum or posterior thoracic cavity may also be complicated by a relatively normal location of the great vessels, making anterior exposure more difficult. Finally, note that goiters extending into the posterior thoracic cavity are extrapleural and behind the lung, which also makes anterior exposure more difficult.
Thyroidectomy for substernal goiter is contraindicated in patients who are inappropriate candidates for surgery in general (eg, patients who have conditions that carry significant or more serious comorbidity). Many practitioners consider the known presence of anaplastic carcinoma of the thyroid a relative contraindication to surgery for substernal goiters because treatment of this condition often is futile, and the likelihood of malignant invasion of critical structures (including the great vessels) may be high. Finally, while toxic substernal goiters may ultimately require surgery for cure, hyperthyroidism requires preoperative medical management to avoid thyroid storm intraoperatively.
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