Anatomy
The thyroid gland is in the midline of the neck and is made up of the isthmus and two lobes (Figure 36.1). Some patients may also have a pyramidal lobe which is a normal developmental variant resulting from an embryological derivative of the thyroid gland. In the fetus, the gland is formed at the junction of the anterior two-thirds and posterior third of the tongue. During intrauterine development, it descends down through the neck (through the region of the hyoid bone) to lie in its anatomical position. It takes with it a tract which obliterates in most people; if this does not close completely, a thryoglossal duct cyst may develop (see Chapter 29). Closely applied to the thyroid gland are the four parathyroid glands which produce parathyroid hormone. This is essential for calcium homeostasis. The gland is underneath the strap muscles of the anterior neck; these muscles help to move the larynx on swallowing. It overlies the trachea centrally, with the lobes of the gland wrapping round the sides to lie over the larynx and cricoid cartilages and the oesophagus and pharynx. The groove that lies between the trachea and the oesophagus is an important landmark as this is the location of the recurrent laryngeal nerve (see Chapter 31). The nerve is susceptible to damage in this region during thyroid surgery, which may result in a weak or breathy voice. A normal-sized thyroid gland is not usually visible but may be palpable; therefore, unless the thyroid is enlarged (a goitre) you will not see it externally in the neck (Figure 36.2).
Physiology
The thyroid gland is stimulated by thyroid stimulating hormone (TSH) from the pituitary gland to produce the thyroid hormones thyroxine (T4) and tri-iodothyronine (T3) by metabolism of dietary iodine. TSH has a negative feedback relationship with T3 and T4 – as the levels of T3 and T4 rise, secretion of TSH by the pituitary is inhibited and vice versa. The thyroid hormones are important in regulating growth and development both in utero
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