Isthmus of Thyroid Gland

The structure indicated is the isthmus of the thyroid gland.

The thyroid gland sits anteriorly in the neck, just inferior to the thyroid cartilage. It is located deep to the overlying strap muscles (sternohyoid, sternothyroid and omohyoid). It has two lateral lobes connected in the centre by the isthmus.

The isthmus lies over the 2^nd and 3^rd rings of the trachea

The lateral lobes extend from the lateral aspects of the thyroid cartilage down as far the 6^th ring of the trachea.

Occasionally a pyramidal lobe is present, which is an upward extension of the isthmus, and is a remnant of the embryological development of the thyroid gland from its descent from the floor of the pharynx.

Blood supply

The thyroid gland is supplied by three arteries:

1. Superior thyroid artery
2. Inferior thyroid artery
3. Thyroid ima artery

The thyroid gland is drained by three veins:

1. Superior thyroid vein
2. Middle thyroid vein
3. Inferior thyroid veins

The parathyroid glands sit on the deep surface of the lateral lobes. There are usually four, with a superior and inferior parathyroid gland lying on either side.