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K. Orhan (ed.)Ultrasonography in Dentomaxillofacial Diagnosticsdoi.org/10.1007/978-3-030-62179-7_19
19. The Thyroid Gland and Ultrasound Applications
ThyroidUltrasonographyFNACCore biopsyCytopathologyNodulesThyroid cancersCervical lymph nodesMetastasesParathyroidParathyroid adenomaFourth branchial cleft cystThyroglossal duct cyst
Normal appearance of the thyroid gland is of a butterfly in the midline of anterior neck. It has two lobes connected by an isthmus. 40% of people would have a pyramidal lobe arising from the isthmus towards the hyoid bone.
The echotexture of the normal thyroid gland parenchyma is homogeneous throughout, higher in echogenicity than the overlying strap muscles. It is similar to the echotexture of a normal parotid gland parenchymal.
Anterior: Strap muscles; Sternothyroid and sternohyoid muscles.
Lateral: Sternocleidomastoid muscles, Superior belly of omohyoid, Common carotid artery (CCA).
Posterior: Oesophagus tends to be on the left, rarely on the right, longus colli, parathyroid gland (only seen if this is enlarged). Some level VI nodes can be visualized.
Medial to the lobe is the trachea.
Blood supply: Superior and inferior arteries; superior, middle, and inferior thyroid veins.
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 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.
19.2 Thyroid Anomalies
19.2.2 Aberrant Thyroid
19.3 Masses in the Midline Level 6
19.3.1 Thyroglossal Duct Cyst
19.3.2 Lymph Nodes
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.
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).
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  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.