Lobes and isthmus
The thyroid gland is composed of two lobes that are usually connected by a narrow centrally-located tissue band called the isthmus. The isthmus is adherent to the upper anterior aspect of the trachea (Read more!) and is placed anterior to the second and third (or fourth) tracheal rings. It is a relatively small structure, and the lobes comprise most of the gland's volume.
The thyroid lobes are sometimes referred to as lateral lobes. Both lobes are located in the lower aspects of the anterior neck triangles. Overall, the tissue of a thyroid lobe is positioned within two minor triangles - muscular and carotid triangles.
Each lobe has two poles - superior and inferior. The superior pole is usually narrower than the inferior pole, and this characteristic gives a somewhat pear-like shape to the lobe. The thyroid lobe measures approximately 1.58 to 2.36 inches (4 to 6 cm) in length. It typically extends from the oblique line of the thyroid cartilage of the larynx to the fourth or fifth tracheal cartilage.
The posterior or posterolateral aspect of each thyroid lobe typically presents with a pyramidal-shaped extension referred to as the Zuckerkandl tubercle. These projections serve as important surgical landmarks during thyroid gland surgery due to their close proximity to the recurrent laryngeal nerves.
Overall, the thyroid gland has a wide variety of anatomical variations. Its lobes may vary significantly in shape, size and symmetry. Therefore, the thyroid gland can appear not only H-shaped but also U-shaped. Some variations with absent isthmus may also be present. In this case, the gland appears as two separate lobes.
Note: Ectopic thyroid tissue is a common finding, and it can be situated anywhere along the embryologic migration sites in the form of various nodules and cysts. Most commonly, ectopic thyroid tissue is found at the base of the tongue.