Anatomy and physiology of the thyroid gland

The thyroid is one of the largest endocrine glands in the body. It controls how quickly the body burns energy, makes proteins, and how sensitive the body should be to other hormones.

Anatomy
The thyroid is composed of two pear shaped lobes which extend from the thyroid cartilage to the 6th tracheal ring on the anterior side of the neck. It lies over the trachea and is covered by layers of pretracheal fascia (allowing it to move), muscle and skin. The two lobes are joined by the isthmus which overlies tracheal rings 2, 3 and 4. The thyroid may enlarge substantially during pregnancy and when affected by a variety of diseases.

Surface anatomy
The normal thyroid gland is easily palpable. Palpation is carried out from behind using the digits to feel for the cricoid cartilage and for the 1st tracheal ring directly below it. The isthmus of the thyroid overlies the 2nd through the fourth tracheal rings, to which the pretracheal fascia (a fibrous sheath that contains the thyroid and allows it to glide smoothly over the nearby contents) firmly attaches through suspensory ligaments (extensions of the fascia). This attachment allows the thyroid to move with the larynx during swallowing, an important fact in palpating the thyroid as it is appropriate to ask the patient to sip a glass of water while palpating the gland, as to allow the inferior portion to be better felt when it elevates with the larynx.

Depending on the state of the thyroid the superior pole is commonly located at the level of the cricothyroid ligament (membrane). While the inferior poles are often difficult to palpate as the sternocleidomastoid relatively speaking) lies just lateral. Two other muscles overlie the majority of the lateral lobes – the sternohyoid and deeper the sternothyroid – however these muscles are small and flat and should not interfere with palpation if the patient is asked to relax the neck (nevertheless the sternothyroid is important in limiting the spread of the thyroid when enlarged).