14 Thyroid Gland

Both processes, then, are responsible in different ways for the development of a goiter.

Ultrasonography is the method of first choice for imaging the thyroid and for evaluating the lymph nodes in the neck and thyroid region. MRI and CT are useful only in selected investigations. Radionuclide scanning (scintigraphy) is best for evaluating focal thyroid lesions and investigating hyperthyroidism.

shoe are formed by the lobes, which are united by the thyroid isthmus. The borders of the thyroid are smooth owing to the presence of the thyroid capsule.

Size. The size of the thyroid gland is determined sonographically by measuring each lobe separately in a transverse scan on the right and left sides (Fig.14.2). This method yields the following approximate dimensions:1 AP depth 10–17 mm, transverse diameter 12–20 mm, longitudinal dimension of each lobe 40–70 mm.

Volume. The volume of the thyroid gland is calculated using the volume formula for a rotational ellipsoid: length of one lobe × width of the lobe × depth × 0.5.

Each thyroid lobe is measured separately and the results are added together to obtain the total volume (the volume of the isthmus is ignored). The normal values for age are shown in Table 14.1. The volume calculation is only an approximation, as it overestimates smaller volumes and underestimates larger volumes. It is difficult to make a reasonably accurate volume measurement in a nodular goiter with poorly defined margins.

Unilateral aplasia is occasionally detected incidentally in adults (previous surgery should be excluded).

Microanatomy of the Thyroid Gland

The microanatomy of the thyroid gland is determined by the follicles. The microfollicles, macrofollicles, and normal-sized follicles range from 50 to 200 μm in size and contain colloid, which consists mainly of thyroglobulin and deposits of monoiodotyrosine and diiodothyrosine. The iodination of thyroglobulin by inorganic iodide to monoiodothyrosine and diiodothyrosine, and probably their coupling to tetra- iodo-thyronine (thyroxine) and triiodothyronine, are catalyzed by the enzyme thyroid peroxidase (TPO).

The single row of epithelial cells in the follicles consists of thyrocytes and calcitonin cells. Even the normal thyroid gland contains thyrocytes, isolated or in aggregates, that are primarily autonomous cells, i. e., not subject to the feedback control system. These cells can proliferate, especially in an iodine-deficient state, and grow into autonomous nodules (adenomas).

Fig. 14.1 Cross-section through the neck at the level of the thyroid gland.

Location of the thyroid gland

●Anterior to the first and fourth tracheal rings

●Isthmus and pyramidal lobe at the level of the cricoid cartilage

●Thyroid lobes anterior in the thyroid trigone

●Relates posteriorly to the pharynx and upper esophagus

Location of the parathyroid glands

●Outside the thyroid capsule

●Upper parathyroids at the level of the cricoid cartilage between the trachea and esophagus, on the posterior aspect of the upper third of the thyroid

●Lower parathyroids on the trunk and between the branches of the inferior thyroid artery; on the lateral or posterior aspect of the inferior thyroid pole or at a lower level

The thyroid gland has a horseshoe shape in anatomical cross-section. From anterior to posterior are the cervical fascia followed by the sternohyoid and sternothyroid muscles, the omohyoid muscles anterolaterally, and the

Fig. 14.4 Longitudinal color duplex scan of the right, structurally normal thyroid gland (TH, cursors). The less echogenic anterior neck muscles are visible anteriorly. Normal, scant color spots appear at a low pulse repetition frequency (PRF) corresponding to a flow velocity of 0.06 m/s.

powerful sternocleidomastoid muscles laterally. Posterior to these muscles are the isthmus of the thyroid gland and the left and right thyroid lobes. They are followed posteriorly by the trachea, parathyroid glands, esophagus (a little to the left), and retropharyngeal connective tissue. Behind that, placed lateral to the spinal column, are the longus colli muscles. The relations noted in this anatomical cross-section can be appreciated in the corresponding transverse ultrasound scan and longitudinal scans through each thyroid lobe (Fig.14.3, Fig.14.4). The portion of the upper esophagus behind the thyroid gland is visible posteriorly between the left thyroid lobe and the tracheal acoustic shadow (Fig.14.5).

Vessels. Ultrasound can consistently demonstrate the blood vessels that run in and around the thyroid gland. The common carotid artery and internal jugular vein run lateral to the thyroid. The superior and inferior thyroid arteries that supply the organ are consistently defined at its upper and lower poles in longitudinal scans (Figs. 14.6, 14.7, 14.8).

Parathyroid glands. The four parathyroid glands show frequent variations in their num-

Fig. 14.5 Left lobe of the thyroid gland lying anterior to the cervical part of the esophagus (OE).

a Transverse scan. See also Fig. 14.10.