This MRCS Part B station tests your knowledge of the anatomy of the thyroid gland and surrounding structures.
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The anatomy of the thyroid gland is commonly tested in the MRCS, especially the blood supply.
The thyroid glands sits in front of the trachea in the pretracheal fascia. It has an interesting blood supply. The superior thyroid artery, a branch of the external carotid artery supplies the upper pole, whilst the inferior thyroid artery from thyrocervical trunk, itself from the subclavian artery supplies the lower pole, and the posterior part of the gland. The isthmus is supplied by the thyroid ima artery in around 10% of people, this artery coming directly off the aorta or brachiocephalic trunk.
The recurrent laryngeal artery runs in close proximity to the inferior thyroid artery therefore it is at risk during ligation of this artery in surgery. Damage to this nerve causes vocal cord paralysis, leading to a hoarse voice.
The upper and lateral parts of the thyroid gland are drained bilaterally by the superior and middle thyroid veins which drain into the ipsilateral internal jugular vein. The inferior thyroid vein drains the lower part into the brachiocephalic vein.
The vocal cords are controlled by the intrinsic muscles of the larynx. Th posterior cricoarytenoid open the cords, the lateral cricoarytenoids close the cords during swallowing, the thyroarytenoids reduce tension in the cords, whilst the cricothyroids increase the tension in the cords. All are supplied by the recurrent laryngeal nerve, except for cricothyroid which is supplied by the external laryngeal nerve a branch of the superior laryngeal nerve.
The internal laryngeal nerve supplies sensation to the larynx above the vocal cords
The external laryngeal nerve supplies the cricothyroid muscle
The recurrent laryngeal nerve supplies all muscles of the larynx except for cricothyroid, and the sensation of the larynx below the vocal cords.