More than 100 cases have been reported in which only one lobe of the thyroid is present (73). The left lobe is absent in 80% of these patients. Often, the thyroid lobe that is present is enlarged, and both hyperthyroidism and hypothyroidism have been reported at times. Females are affected three times as often as males. Both benign and malignant nodules have been reported in this condition (74).

Other variations involving the median thyroid anlage represent an arrest in the usual descent of part or all of the thyroid-forming material along the normal pathway. Ectopic thyroid development can result in a lingual thyroid or in thyroid tissue in a suprahyoid, infrahyoid, or intratracheal location. Persistence of the thyroglossal duct as a sinus tract or as a cyst (called a thyroglossal duct cyst) is the most common of the clinically important anomalies of thyroid development. Finally, the entire gland or part of it may descend more caudally; this results in thyroid tissue located in the superior mediastinum behind the sternum, adjacent to the aortic arch or between the aorta and pulmonary trunk, within the upper portion of the pericardium, and even within the interventricular septum of the heart. Most intrathoracic goiters, however, are not true anomalies, but rather are extensions of pathologic elements of a normally situated gland into the anterior or posterior mediastinum. Each of these abnormalities is discussed in greater depth.