19.1 Size of the Thyroid Gland

Size of the thyroid is normally measured in volume with measurement taken in three dimensions. It is easier to use a probe which is larger than a 6 cm linear probe. This would enable to scan the thyroid lobe in its maximum dimension. Remember to freeze the US image before measuring. The three dimensions to measure are width, depth, and length. It is not compulsory to measure the thyroid if the gland appears to be of normal size and echotexture. Size is important when there are nodules or if the thyroid is seen to be pathological.

The transverse dimension (width-W) and anterior-posterior (depth- D) dimension is approximately 2 cm (Fig. 19.2).

The length (L) of the lobes is 4–5 cm. Measured longitudinally (Fig. 19.2).

A quick way to check for enlargement is the look at the width of the isthmus. Any enlargement of >0.5 cm generally indicates enlargement of the thyroid gland.

A more detailed consistent way to assess the thyroid size is by measuring the volume. The volume is measured using π/6 (W × D × L). Most US machine has an onboard computer to calculate this when capturing the measurements [1]. The isthmus is normally ignored when calculating the volume unless there is a large nodule present within the isthmus. The normal size is approximately 10 ml (Fig. 19.3).

19.2 Thyroid Anomalies

19.2.1 Hemigenesis

The incidence is 1:2500 with 96% involving hemigenesis of the left lobe [1]. It is a benign condition and often is an incidental finding when scanning for something else (Fig. 19.4). Double check that patient has no surgical scar present in the vicinity.

19.2.2 Aberrant Thyroid

Thyroid developed and descend from the base of the tongue. It descends below the larynx, which bifurcates into two lobes connected with an isthmus [1]. Several scenarios can occur. There may be an incomplete descend, disruption to the descend, or descend into the mediastinum. There may also be a lateral aberrant thyroid tissue in the neck. It is important to evaluate this to rule out a metastatic lymph node from a thyroid origin (Fig. 19.5).

19.3 Masses in the Midline Level 6

19.3.1 Thyroglossal Duct Cyst

Arises from embryonic tissue from the foramen caecum at the base of the tongue to the larynx. Portion of the duct near the isthmus may persist as a pyramidal lobe of the thyroid. This may be filled with proteinaceous material secreted by the lining epithelium (Fig. 19.6). Malignancy changes can occur in long standing Thyroglossal duct cyst, this is said to be approximately 1%.

19.3.2 Lymph Nodes

Enlarged inflammatory lymph nodes are frequently seen on ultrasound. This may be reactive nodes due to variety of causes (Fig. 19.7).

19.3.3 Malignant Masses

Most frequently found are metastasis from occult primary cancer of head and neck, the commonest is squamous cell carcinoma followed by lymphoma.

If any abnormal mass is found in the neck, ultrasound-guided fine needle aspiration cytology (FNAC) is highly recommended for definitive diagnosis (Fig. 19.8).

19.3.4 Parathyroid Cyst

This is rare. More commonly found in female and can range from 1–6 cm. This is fluid-filled sac that can be located posterior or inferior to the thyroid gland or in the mediastinum. Patient may or may not be symptomatic (Fig. 19.9a, b, c).

19.3.5 Parathyroid Adenoma

Parathyroid adenoma can be found in patient who has primary hyperparathyroidism, MEN1 and MEN2a syndromes, and chronic renal failures. On US this appears to be hypoechoic with heterogeneous echotexture, some with cystic degeneration. This may have internal vascularity (to differentiate from a node)—but not always. (Fig. 19.10a, b) The best investigations to confirm a parathyroid adenoma are a combination of Tc99m Sestamibi and US, together with a positive serology of increased calcium level and parathyroid hormone (PTH). US is extremely good for precise localization of the parathyroid adenoma. The advantages of the usage of US for localization are the ease of availability and safety of US, lack of ionizing radiation, patient’s comfort, short imaging appointment time, and cost-effective to the patient and the health system. The disadvantages includes operator experience and limitations.

Positioning of the patient is very important when scanning for parathyroid adenoma. At least a pillow or two should be placed underneath the upper body and shoulders of the patient to allow the head to be tilted back and to fully extend the neck. It may not be possible to do this with a patient who suffers from ankylosing spondylosis or if they have neck problems.

Parathyroid adenoma may be located superior or inferior to the thyroid lobe, within the thyroid gland, along the tracheoesophageal groove or intrathoracic, it is important to be able to US the neck by turning the patient’s head from side-to-side and by asking the patient to hold their breath or cough.

19.3.6 Parathyroid Carcinoma

This tends to be hypoechoic and tends to be inferior to the thyroid lobe. This can also be found within the thyroid gland. It is indistinguishable from a parathyroid adenoma. It tends to have a thicker capsule.

19.4 Fourth Branchial Cleft Cyst/Cervical Thymic Cyst

The fourth branchial cleft cysts are very rare. They can form a sinus from the apex of the piriform fossa to the anterior upper pole of the left thyroid lobe. They appear to run parallel to the recurrent laryngeal nerve. 80% of cyst appears on the left (Fig. 19.11).

19.5 Thyroiditis

Chronic autoimmune thyroiditis and Graves’ disease are two forms of autoimmune thyroid disease (AITD). Hashimoto thyroiditis and Graves’ disease have been commented as being the same autoimmune thyroid disease but at the different end of the spectrum. Their appearance in US are similar. The typical difference is Graves’ disease displayed hypervascularity with power Doppler analysis. The vascularity of Hashimoto thyroiditis is variable and can range from avascular to hypervascular; however, the peak systolic velocity (PSV) tends to be below that of Graves’ disease. It has been suggested that if PSV for the superior thyroid artery is >50.5cm/s, this is more likely to be for Graves’ disease (Figs. 19.12, 19.13, and 19.14).

19.6 Thyroid Nodules

US is an extremely sensitive imaging modality to detect thyroid nodules which can be as small as 2–3 mm. The prevalence of detecting nodules are 50–60% in comparison to palpation [2] Mazzaferri. There is a steady increased rise in the detection of thyroid nodules due to increased sensitivity of our imaging modalities since 1980s with widespread use of US, and in 1990s with increased use of CT and MRI. More sub centimeter nodules are being detected and investigated; however, the mortality from thyroid cancer has not changed since 1980s. It is very important to be able to characterize and recognize a benign thyroid nodule from a malignant nodule. There are several thyroid guidelines produced by eminent thyroid associations around the world and readers are recommended to consult them. As more US studies are being published, certain US features and characteristics have been proven to be helpful in differentiating a malignant nodule from a benign one. With this, come several risk stratifications purposed by various thyroid associations.

19.7 US Characteristics of Thyroid Nodules: What to Look for and What to Include in Our US Thyroid Report