Gland
Isthmus of the thyroid
Variable bridge of thyroid tissue across the upper trachea
High-yield
The second to fourth tracheal rings are a useful approximate landmark, not a fixed boundary. Human dissections show several overlapping levels.
Overview
The isthmus joins the thyroid lobes in front of the trachea, beneath the infrahyoid muscles. Its height, thickness and relation to the tracheal rings differ among individuals, and it may be absent. A pyramidal lobe can extend upward from this bridge.
Reference facts
- Location
- Across the anterior upper trachea, usually near the second to fourth rings, with substantial variation.
- Hormones
- Its thyroid follicles make T4 and T3, as do follicles in the lobes.
- Blood supply
- Communicating superior and inferior thyroid arterial branches; a thyroid ima artery may contribute.
- Innervation
- Autonomic fibers accompany its vessels. Follicular activity is regulated principally by TSH and thyroid-hormone feedback.
- Function
- Contributes functioning thyroid tissue and connects the lobes when present; it has no separate endocrine task.
Clinical correlation
The isthmus and nearby pretracheal veins are relevant to bleeding during anterior airway procedures. A thyroid ima artery may also occupy this region. Their anatomy varies, so a remembered ring level does not establish a safe access route.
Frequently asked questions
Where is the isthmus of the thyroid located?
The isthmus of the thyroid lies across the anterior upper trachea, usually near the second to fourth rings, with substantial variation.
What does the isthmus of the thyroid secrete?
Its thyroid follicles make T4 and T3, as do follicles in the lobes.
What does the isthmus of the thyroid do?
Contributes functioning thyroid tissue and connects the lobes when present; it has no separate endocrine task.
What is the blood supply of the isthmus of the thyroid?
Communicating superior and inferior thyroid arterial branches; a thyroid ima artery may contribute.