Shape and dimensions

In the frontal plane the thyroid is shaped like an H or a butterfly (Fig. 22.1). In the transverse plane it has the form of a horseshoe, concave posteriorly and enclosing the trachea. The right and left lobes are approximately conical and connected by a narrow medial isthmus.

Fig. 22.1 (A) Frontal view and profile of the thyroid. (B) Posterior view of the thyroid.

Frequently larger in women than in men, the lobes generally measure:

The thyroid usually weighs 30 g, but this varies. It is brownish-red in color. Its consistency is rather soft, and its parenchyma is slightly granular. Its surface, when lightly palpated, appears somewhat irregular.

The thyroid

Stability

The thyroid parenchyma is surrounded by a thin fibrous capsule, a dependency of the visceral sheath of the neck. The capsule is held in place by ligaments of the trachea and the vascular sheath called the thyrotracheal ligaments, one medial and two lateral. These are also referred to as the ligaments of Grüber.

The thyroid compartment is in the shape of a U, open at the back. The thyroid sheath enclosing it is made up:

The pretracheal lamina inserts on the inferior border of the hyoid bone and splits into two layers:

Physiology

The thyroid gland secretes two hormones that stimulate cellular metabolism, and one hormone concerned with phosphate and calcium metabolism.

Description

The parathyroid glands are small endocrine glands situated on the posterior surface of the lateral thyroid lobes. They are paired glands, divided into two groups: the superior and inferior parathyroids.

Each gland is commonly 8 mm long, 4 mm across, and 2 mm from back to front. Each weighs about 40 mg.

The superior parathyroid glands are attached to the surface of the apex of the thyroid, near the external branch of the superior laryngeal nerve.

The inferior parathyroid glands are attached to the caudal edge of the thyroid lobes, each near an inferior thyroid artery and a recurrent laryngeal nerve.

Physiology

The parathyroids secrete parathyroid hormone (parathormone) and play an important role in calcium metabolism. Secretion is regulated by blood calcium levels. When calcium concentration falls, parathyroid hormone secretion increases, and vice versa.

The main function of the parathyroid hormone is to maintain a constant concentration of calcium in the extracellular fluid. Thus, when blood calcium levels falls, parathyroid hormone:

Inspection

Visual inspection can provide precious clues. This may be noticed with a quick glance, or with specific observation such as a slight extension of the neck or on swallowing.

The thyroid is sometimes visible even in the absence of pathology. In subjects with elongated graceful necks, the isthmus region can be clearly seen. Sometimes the thyroid can be noticed casually in profile, as a discrete anterior cervical bulge that moves with swallowing (Fig. 22.2).

Fig. 22.2 Frontal view of the neck.

Ordinarily, everyone has a slight depression at the lower part of the neck, between the sternocleidomastoid muscles. Its spontaneous disappearance on rotation of the head is a good indication of an enlarged thyroid.

Pay attention to small, unusual projections on the thyroid at the median line or behind the sternocleidomastoid muscle. Any mass that moves during the course of swallowing is highly likely located in the thyroid gland, or adhered to it.

Should you observe hypertrophy, notice whether it is diffuse, like a goiter, or localized, like a nodule or thyroid cyst.

Palpation

The superficial location of the thyroid suggests that palpation will be easy. However, in the absence of pathology, palpation of the gland is hampered by the thickness of the infrahyoid and sternocleidomastoid muscles covering it. Ideally, palpation is done with a patient seated with their back leaning against the therapist, who stands behind the patient. To begin, either the patient’s head is in the standard neutral position or the cervical thoracic junction is slightly flexed to reduce tension on the anterior neck muscles. With practice it is possible to perform this palpation in supine.

You might want to provide a glass of water for the patient if he or she needs to swallow repeatedly.

Identify the cricoid cartilage and then, with the pad of your index and middle fingers, search caudally to feel the isthmus, located about 1 cm below.

To palpate the isthmus from the sternal notch, glide your fingers upwards using a light touch, and short back and forth movements. Your fingers will run into the substance of the isthmus. Have the patient swallow and feel the elastic isthmus elevate under your finger (Fig. 22.3).

Fig. 22.3 Larynx and thyroid in profile.

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