Thyroid

The thyroid is an endocrine gland concerned with metabolic activity. It is situated in the anterior neck and its two lobes lie on either side of the laryngotracheal junction. The lobes are united across the midline by a short isthmus that passes anterior to the second and third tracheal rings. Each lobe is 5 x 3 cm and is related laterally to the carotid sheath. The four parathyroid glands (each 3–6 mm across; are endocrine glands concerned with calcium metabolism) are related to the posterior surface of the thyroid (two to each lobe) and enclosed within its fascial sheath. Due to its fascial attachments, the thyroid gland moves with the larynx on swallowing; it is not usually visible as it is covered anteriorly by the infrahyoid and anterolaterally by the sternomastoid muscles.

Enlargement of the gland (goiter) may be diffuse, as with iodine deficiency and the firm gland of Hashimoto’s disease, or be due to one or more nodules The latter are usually cysts but may be benign or malignant neoplasms (figure 47). Enlargement may compress the trachea, and stretch, invade and paralyse the recurrent laryngeal nerve: vocal cord paralysis may be observed by direct or indirect laryngoscopy.

Thyroid over-activity (thyrotoxicosis) is accompanied by hyperactivity, anxiety, weight loss, weakness, tremor and abnormal eye signs (see below). There is tachycardia, and this may be accompanied by auricular fibrillation and cardiac failure. Under-activity (myxedema) is characterised by slow mental activity, fatigue, lethargy, weakness, dry skin and hair, facial edema, loss of the lateral eyebrows, slow reacting reflexes and, more rarely galactorrhea, pericardial effusion, heart failure, myxedema madness, coma and hypothermia.

The developing thyroid descends from the back of the tongue, and remnants of this tract may persist as a thyroglossal cyst.

Examination of the thyroid commences with observation from the front and the side, looking for enlargement and asymmetry of the lower neck. Observe the thyroid during swallowing. The subject takes a mouthful of water, holds it, extends the neck and then, when requested, swallows; enlargement is more obvious as the gland moves upwards on swallowing. Although the female larynx does not have a laryngeal prominence (Adam’s apple), enlargement is equally visible on swallowing (figure 48a- d).

The gland is more easily palpable from behind with the chin slightly down and the neck muscles relaxed (figure 49a). First palpate the isthmus in the midline over the upper tracheal rings, then each lobe in turn. Pushing the larynx to one side makes it easier to palpate the opposite lobe (figure 49b). When examining from in front, the same laryngeal deviation makes the opposite lobe more prominent and palpable (figure 49c).

Check the position of the trachea, as it may be deviated to one side as well as compressed (figure 50).

Enlargement of the thyroid may be into the superior mediastinum. It rises on swallowing, and may also be detected by percussion across the manubrium (figure 51a). A hyperactive gland may have an audible bruit (figure 51b). The bell or the diaphragm of the stethoscope is lightly applied to avoid compression and artifactual production of murmurs from a carotid artery.

Examine the neck for enlarged lymph nodes (see – Cervical Nodes) that may be thyroid metastases.

The exophthalmos of thyrotoxicosis may produce lid retraction with increased visibility of the sclera above (and less commonly below) the iris; the protrusion may be more obvious when observed from the side or from above. There may be lid lag: this is demonstrated by asking the subject to follow a finger that is slowly lowered from above downwards, noting delay in dropping of the upper lid (figure 52a,b). Also pass your finger laterally to note any abnormalities of gaze or differences between the two sides and any nystagmus in the lateral extreme (figure 52c).

Examine the hands and note any excess sweating, tachycardia or tremor. Ask the subject to hold their hands out straight to observe tremor; this can be accentuated by placing a sheet of paper on the dorsum of the outstretched hands (figure 53).

When examining a thyroglossal cyst, ask the subject to put out their tongue to demonstrate the upward pull from its primary developmental attachment.

Figure 54 Cervical incisions

 

1. Block dissection of neck
2. Parotid gland
3. Submandibular gland
4. Cervical node sampling/Carotid endarterectomy
5. Cricothyroid puncture
6. Thyroidectomy
7. Tracheostomy (transverse)
8. Cervical sympathectomy/subclavian artery
9. Tracheostomy (vertical)

The thyroid gland is approached through a transverse (collar) incision, two centimetres above the clavicle; this passes over part of each sternomastoid muscle. A narrower transverse or vertical incision provides access to the upper trachea for tracheostomy, separating the infrahyoid muscles. The tracheostomy tube is passed into a circular hole or a flap made in the upper trachea. This is usually through the third and fourth rings, taking care to retract or divide the isthmus of the thyroid and control any bleeding from its associated vessels. An incision through the cricothyroid membrane (cricoidotomy) enters the trachea below the vocal cords and is an alternative approach in children. In an emergency a wide bore needle is passed through the cricothyroid membrane to obtain an airway.

Excision of a thyroglossal cyst is through a transverse incision over it. The complete tract must be removed, and this may require excision of the central portion of the body of the hyoid bone.

Exposure of the carotid bifurcation and common carotid artery is by an incision along the anterior border of the sternomastoid muscle. The upper part of this incision also exposes the upper part of the deep cervical lymph chain.

The parotid gland is approached through a vertical incision along the front of the ear, extended under the lobe and along the upper anterior border of the sternomastoid muscle.The surgical approach to the submandibular gland is through an incision parallel with the lower jaw, but two centimetres below it, to avoid damaging the mandibular branch of the facial nerve that innervates the lower facial muscles.