Isoechoic

Normofollicular Adenoma Hemorrhagic or Colloid Cyst

Regressive Changes in an Adenoma or Adenomatous Nodule

Parathyroid Adenoma/Hyperplasia Lipoma

As a rule, solitary isoechoic nodules in the thyroid gland are normofollicular adenomas. They have the same echo structure as the rest of the thyroid parenchyma and can be distinguished only by their hypoechoic or anechoic rim (halo). Normofollicular thyroid nodules, like hypoechoic nodules, may show increased radiotracer uptake at scintigraphy, identifying them as autonomous adenomas. By contrast, hyperechoic adenomas almost never show uptake that indicates autonomy.

Sonographic features. Normofollicular adenomas have the same reflectivity as normal thyroid tissue, with the result that adenomas usually are not detectable with ultrasound unless they are located at the periphery of the gland and cause an appreciable contour bulge. They may be circumscribed by an echo-poor vascular rim (Fig.14.48, 14.3b).

Fig. 14.48 Isoechoic adenoma (A) in a euthyroid patient. M = muscle.

a B-mode image shows an elliptical halo in an otherwise normal-appearing thyroid (TH).

b Color Doppler demonstrates a vascular rim with slight internal vascularity.

Besides adenomas, isoechoic nodules may be caused by fresh bleeding within a cyst or by large colloid-containing cysts. These cystic lesions can be identified as such by noting their compressibility at ultrasound. Subtle movements of the cyst contents may also be observed. FNA can quickly resolve any doubts as to whether the mass is liquid or solid (Fig.14.49, 14.2f). Carcinomas are rarely isoechoic and are almost never hyperechoic.

Fig. 14.49 A cyst with internal hemorrhage is predominantly isoechoic and creates a bulge in the thyroid contour. Fine-needle aspiration identified this lesion as a chocolate cyst.

Parathyroid adenoma and hyperplasia are predominantly hypoechoic but also isoechoic and rarely hyperechoic (regression). They are to be found at the usual positions dorsally, laterally of the thyroid poles, and most commonly at the lower poles (Fig.14.51).

Lipoma

Fig. 14.51 Parathyroid adenoma (cursors), located dorsally and close to the aorta (AO).

A lipoma is occasionally misinterpreted clinically as a nodular goiter. While its ultrasound features resemble those of adenoma or microcystic nodular transformation, it can be distinguished (when considered in the differential diagnosis) by its typical mottled or streaky isoechoic appearance and by the lack of vessels in the CDS. The thyroid can usually be clearly delineated from the lipoma embedded in the anterior subcutaneous fat (Fig.14.52).

Fig. 14.52 Patient had received replacement therapy for a goiter for years. A nodule extends far toward the right side. A lipoma is delineated only by its lack of vascularity on color duplex examination of the thyroid gland (TH). AC = common carotid artery.

Hyperechoic

Macrofollicular Adenoma or Nodule Hemorrhagic Cyst, Colloid Cyst Nodule with Regressive Changes

Hemangioma, Myolipoma, Thyroid Regressive Changes Calcifications

Solitary hyperechoic nodules in an otherwise normal thyroid gland are generally macrofollicular adenomas. They show no tracer uptake at scintigraphy, appearing as cold nodules. Scintigraphy cannot evaluate the structural changes that underlie cold nodules, which may represent carcinoma, fibrous regressive nodules, or cysts. Sonography is an ideal complement to scintigraphy in making this kind of differentiation (Table 14.5).

Hyperechoic adenomas are less common than hypoechoic microfollicular adenomas. Macrofollicular nodules show little or no internal vascularity when evaluated by color Doppler.

Nodules with fibrous regressive changes, like macrofollicular adenomas, are more echogenic than normal thyroid tissue but tend to display a coarsely granular to patchy echo texture (Fig.14.53).

Parathyroid adenoma and hyperplasia demonstrate a predominantly hypoechoic structure; in very rare conditions they may also present a very high reflectivity due to regressive changes. As a differential diagnosis, hemangiomas and myomas should also be taken into consideration. When these lesions are found, it is difficult to tell what histopathological elements are responsible for their high echogenicity. They have no real clinical significance (Fig.14.55).

Fig. 14.55 Small, round, hyperechoic nodule (cursors) in a hypoechoic thyroid gland. The differential diagnosis consists of hemangioma, lipoma, and thyroid adenoma with regressive changes. The patient presented with renal failure and mild hyperparathyroidism.