The downward prolongation of the thyroid is due to growth from the lower pole of either a single nodule or one of several nodules in a nodular goiter. When one remembers the anatomy of the thyroid, it is easy to see why at times the growth may be downward rather than anterior, where it is limited by the pretracheal muscles, or posterior or lateral, where it meets resistance from firm structures. The deeper layer of pretracheal muscles is inserted into the posterior part of the clavicle and sternum, and as the nodule slides up and down with deglutition and other movements of the neck, it is guided behind the bony structures of the cervical ring. For a varying length of time, generally a long period, the mass will descend into and come out of the thorax with perfect ease. After a time, more and more of the mass falls below the superior bony margin. The moment the nodule reaches a size that precludes its moving upward into the neck itself, it becomes a completely intrathoracic goiter and, because of its fixed position, is potentially more dangerous in case of sudden swelling from any cause. The size of the growth within the thorax may increase markedly. A tongue of tissue may extend behind the trachea and esophagus, and the lower level may be below the level of the aortic arch and even as far as the diaphragm. Although the lower pole is the usual starting point, it is possible for the growth to start from a lateral lobe and thus add to the difficulties of diagnosis because of the apparent freedom of the lower pole from any connection with the intrathoracic growth.

Substernal and intrathoracic goiters are typically found in older persons. A kyphosis, a stooped posture, and an increased anterior-posterior thoracic diameter no doubt promotes an intrathoracic position of the gland. By having the patient lie supine with a pillow under the shoulders, a large part of the gland may be delivered into the neck, and at surgery it is sometimes surprising how easily a large substernal goiter may be pulled out through the cervical inlet and removed without recourse to splitting of the sternum.

The symptoms are most often due to mechanical factors, the result of pressure from the mass on surrounding structures. Thyrotoxicosis may also arise from such a gland. Often the first thing that bothers the patient is an obstruction to breathing when the head is in a particular position or when he or she is asleep and the head is lying at a fixed angle. Patients may give a history of attacks during the day or night during which they fear they will suffocate. Dyspnea may suddenly become severe during a respiratory infection and can lead to respiratory failure unless recognized. Bleeding into a cystic lesion is also a cause of sudden accentuation of obstructive symptoms. An irritable cough or slight hoarseness may occur. Difficulty in swallowing may develop so insidiously that the patient is not aware of the problem until it has assumed considerable proportions. Patients with large, strategically located masses may show a striking pattern of dilated veins over the upper chest. Very rarely a vena cava superior syndrome may ensue.

A patient with a substernal goiter may show tracheal deviation or evidence of dilated cervical or facial veins. The potential for venous obstruction may be made apparent by having the patient elevate the arms above the head. If external jugular vein dilatation is seen (Pemberton's sign), a significant obstruction to venous return is present due to the mass in the thoracic inlet. The roentgenogram will show a substernal tumor and often deviation of the trachea. Radioisotope scanning (with 131I) is of much help in defining the limits as well as in identifying the nature of the mass. CT or MRI imaging may be necessary especially to distinguish a goiter from a vascular tumor or aneurysm.

The small, asymptomatic substernal goiter does not require therapy. If necessary, 131I treatment can be applied. Surgery is hardly ever necessary, but if so (relatively young patient and or high dose radioiodine needed), these glands do not demand any change in anesthesia or special surgical treatment. Since their greatest diameter is above the level of the strait, they lift out easily and there is no greater danger of nerve damage than in routine thyroidectomy nor is there any increased risk of postoperative complications.

When the growth is lodged below the bony strait, the problem may be more serious. Tracheal intubation ensures the continuity of breathing if it should become necessary to exert pressure against the trachea during the surgical procedure. The blood supply of the mass comes from the regular sources of blood supply to the thyroid and is carried down into the thorax with the descent of the goiter. Consequently, the procedure is first to control the blood supply from above by tying the upper pole and the lateral vessels, and then to find the line of cleavage that will allow the mass to be separated from the bed in which it lies. Because the tracheal tube ensures an adequate supply of air, it is possible, in practically all cases, to deliver the growth with a finger lifting below and traction from above.