Although the intrathoracic thyroid gland is almost always a continuation of the pathological cervical thyroid gland, it is convenient to consider it in this section, since it is a relatively common occurrence and has to be considered in the differential diagnosis of tumors of the anterior and upper mediastinum. A lack of accurate knowledge of the frequency of the intrathoracic thyroid gland has already been noted; if such information is included in the reports, then they indicate 15-20% of the number of observations. A heterotopic thyroid gland, which may even be the only one functioning in a patient, is relatively rare and is a formation in the anterior mediastinum, usually diagnosed only with thoracotomy. This condition differs from the more frequent form of the intrathoracic thyroid gland in that it has no vascular connections with the cervical thyroid gland, although its blood supply can originate in the neck.

Pathological anatomy. Embryonic pathological anatomy of the intrathoracic thyroid gland. The thyroid gland develops from the endoderm of the primary pharynx, as a median outgrowth where the blind fossa of the tongue is ultimately located. The future thyroid gland appears at the 4th week of embryonic development, is closely connected with the aortic sac and grows downward along the full lingual-thyroid duct, the proximal end of which remains in the form of a blind fossa. Abberant thyroid tissue can therefore be found anywhere along the thyroid-lingual duct – from the base of the tongue, then in the upper and anterior mediastinum, up to the pericardium and even in the heart [33]. A heterotopic thyroid gland in the mediastinum, pericardium and heart arises due to the fact that the developing thyroid tissue is displaced into the chest when the heart and large vessels descend. Usually it is located in the anterior mediastinum along the thymus gland. The presence or absence of its connection with the thyroid gland normally located in the neck determines blood supply, which can come from the vessels of the neck or from local blood vessels. 

Pathological changes in diseases. In most cases, the intrathoracic thyroid gland is a simple colloid goiter spreading to the chest area. This occurs with equal frequency in men and women and is most common in middle-aged and elderly people. Toxic effects with signs of hyperthyroidism are rare; malignancy occurs more frequently.

Typically, intrathoracic goiter comes from the lower pole of the lateral lobe, on the right more often than on the left. Therefore, it is located in front of the trachea, in the upper part of the anterior mediastinum and directly behind the sternum. In almost 10% of cases, goiter arises from the posterolateral part of the gland and ultimately is localized behind the trachea and posteriorly and to the side of the esophagus, which gradually shifts more and more as education spreads to the chest. The main factor determining the growth of the cervical goiter in the chest is the restriction of its growth anteriorly and laterally by the muscles of the neck. As soon as the goiter enters the chest, it enters the area of ​​least resistance.

Since the enlargement of the thyroid gland is usually asymmetric, it displaces the more mobile mediastinal masses, in particular the trachea and esophagus, in the opposite direction.

The clinical picture (symptoms and signs). The growth of goiter usually occurs slowly, so that the symptoms associated with pressure develop latently, and it can take many years, during which the patient can reach the stage of severe disability due to shortness of breath, and the reason for this will remain unrecognized. The main blow of compression is taken by the trachea, since the esophagus is more mobile and usually only moves. With the usual type of intrathoracic goiter, dysphagia occurs rarely. Compression of the esophagus with signs of dysphagia in most cases is the result of malignant changes. Compression of the trachea can cause shortness of breath and stridor, as well as erosion of the cartilage rings, so that even after removal of the formation, obstruction of the airways is possible due to the flexibility of the trachea without a frame. Tension of the recurrent laryngeal nerve with a tumor can lead to paralysis of the vocal cords with hoarseness, but much more often this is a complication of the operation. Obstruction of the large veins can cause overflow of the cervical veins, which increases when lying down or when the patient bends.

Usually patients belong to the age group older than 50 years, and often they are obese due to limited mobility. Obesity often interferes with early clinical and radiological diagnosis. Just like shortness of breath, compression of the trachea can cause a painful cough. The course of events can be dramatically accelerated by hemorrhage in the colloidal intrathoracic goiter. There may be an urgent need for emergency surgical treatment to stop breathing problems. Typically, patients with intrathoracic thyroid gland palpable gland. Thoracic growths are sometimes missed with cervical thyroidectomy.

A direct chest x-ray shows a rounded shadow in the upper mediastinum, which is usually anterior in the lateral image. Being asymmetric, the formation, as a rule, swells more in one direction (more often to the right), and the presumptive diagnosis is based on the fact that the upper border of the shadow is poorly defined, continuing into the shadow of the soft tissues of the neck. The trachea is displaced and can be narrowed. With the prolonged existence of adenomas, calcification and cysts can be determined. A study with barium reveals the movement of the thyroid formation when swallowing, and in those few cases when the formation is located behind the trachea, you can see the displacement of the esophagus. To show the presence of functioning glandular tissue in the mediastinum as well as its normal complement in the neck, a study with radioactive iodine can be used. However, this study is not possible in every case, and preliminary treatment with iodine makes this test unsuitable. The clinician should not neglect a negative result, but a positive test is valuable evidence of the presence of thyroid gland tissue in the chest. In some cases, a retrosternal goiter may be taken as an aortic aneurysm or a tumor of the goiter. If aneurysm cannot be ruled out in other ways, angiography may be indicated.

Thoracic goiter may be affected by Hashimoto’s disease. In this case, there is a tendency to the appearance of severe compression symptoms, which arise with carcinomatous changes. Detection of anti-thyroid autoantibodies is proof of diagnosis, and since thyroxine treatment is usually successful, surgery can be avoided.

Treatment. With the possible exception of cases of Hashimoto’s disease, all intrathoracic goiter are subject to surgical removal. A heterotopic mediastinal thyroid is almost always diagnosed only with thoracotomy.