The isthmus is the narrow bridge of thyroid tissue that joins the right and left lobes across the front of the windpipe. It usually lies over the second to fourth tracheal rings, just below the cricoid cartilage.
| Location | Midline of the anterior neck, over tracheal rings 2-4, below the cricoid cartilage and deep to the sternohyoid and sternothyroid muscles |
|---|---|
| Secretes |
|
| Blood Supply | Anastomosis of the right and left superior thyroid arteries along its upper border; Thyroid ima artery (inconstant) |
| Venous drainage |
|
| Lymphatic drainage |
Connects the two thyroid lobes and contains functioning thyroid follicles that contribute to hormone production.
The isthmus sits directly in the path of a tracheostomy and is usually retracted upward or divided between clamps to reach the trachea. A nodule here is easy to feel and can be removed by isthmusectomy. The isthmus is occasionally absent.
A thyroid nodule in the isthmus, assessed with ultrasound and fine needle aspiration like nodules elsewhere in the gland.
Bleeding from the vascular isthmus or the anterior jugular and inferior thyroid veins nearby when it is cut or torn during tracheostomy.
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