Thyroid Gland Anatomy

Anatomy of the Thyroid Gland

• The thyroid is located in the anteroinferior neck at the level of the thyroid cartilage.
• It consists of right and left lobes connected by an isthmus.
• The gland straddles the trachea anteriorly.
• Laterally, it is bounded by the carotid arteries and jugular veins.
• A pyramidal lobe may arise from the isthmus.

Thyroid Size

• Lobes are normally equal in size.
• In newborns, the thyroid measures 18 to 20 x 8 to 9 mm.
• A normal adult thyroid is 40 to 60 x 20 x 13 to 18 mm.
• The isthmus measures 4 to 6 mm in the anteroposterior (AP) diameter.

Volume

• Thyroid volume calculation is commonly based on the ellipsoid formula with a correction factor: length x width x thickness x 0.52 for each lobe.
• The normal mean thyroid volume is 18.6 ± 4.5 ml.
• Thyroid volume in males is slightly larger than in females.

Relational Anatomy - Anterior

• The anterior surface is covered by strap muscles, including sternothyroid, omohyoid, sternohyoid, and sternocleidomastoid muscles.
• Sternohyoid and omohyoid muscles appear as thin, hypoechoic bands anterior to the gland.
• The sternocleidomastoid muscle is a larger oval band lying anterior and lateral to the gland.

Relational Anatomy - Posterior

• The posterolateral anatomy includes the common carotid artery, internal jugular vein, and vagus nerve.
• The longus colli muscle is posterior and lateral to the lobes, presenting as a hypoechoic triangular structure adjacent to the cervical vertebrae.

Relational Anatomy - Medial

• Medially, the thyroid is related to the larynx, trachea, inferior constrictor of the pharynx, and esophagus.
• The esophagus, primarily a midline structure, may be found to the left of the trachea.
• In transverse plane, the esophagus has a target appearance; peristaltic movements occur during swallowing.
• The posterior border of each lobe relates to the superior and inferior parathyroid glands, and there's anastomosis between the superior and inferior thyroid arteries.

Blood Supply

• Two superior thyroid arteries arise from the external carotids and descend to the upper poles.
• Two inferior thyroid arteries arise from the thyrocervical trunk of the subclavian artery and ascend to the lower poles.
• Doppler peak systolic velocities reach 20 to 40 cm/sec in major thyroid arteries and 15 to 30 cm/sec in intraparenchymal arteries.
• Corresponding veins drain into the internal jugular veins.

Thyroid Physiology and Laboratory Data

• The thyroid maintains normal body metabolism, growth, and development by synthesizing, storing, and secreting thyroid hormones.
• The mechanism for producing thyroid hormones is iodine metabolism.
• The thyroid gland traps iodine from the blood and through a series of chemical reactions produces triiodothyronine (T3) and thyroxine (T4).
• When thyroid hormone is needed, the gland is stimulated by thyrotropin, or thyroid-stimulating hormone (TSH), produced by the pituitary gland, this causes its release into the bloodstream.
• Calcitonin decreases the concentration of calcium in the blood by inhibiting bone breakdown and helps maintain blood calcium homeostasis.

Euthyroid

• A thyroid producing the correct amount of thyroid hormone is considered normal, or euthyroid.

Hypothyroidism

• Undersecretion of thyroid hormones is hypothyroidism.
• Causes of hypothyroidism include low iodine intake (goiter), inability of thyroid to produce hormone, or issues with pituitary gland's control.
• Clinical signs and symptoms include myxedema, weight gain, hair loss, increased subcutaneous tissue around eyes, lethargy, intellectual and motor slowing, cold intolerance, constipation, or deep husky voice.

Hyperthyroidism

• Oversecretion of thyroid hormones defines hyperthyroidism.
• Hyperthyroidism results from an out-of-control entire gland or localized neoplasm (adenoma).
• This condition dramatically increases metabolic rate; clinical signs include weight loss, increased appetite, high degree of nervous energy, tremor, excessive sweating, heat intolerance, and palpitations; many patients experience exophthalmos (protruding eyes).

Tests of Thyroid Function

• Nuclear medicine is used to determine thyroid function through iodine uptake and thyroid scans.
• Lab tests measure the amount of T3 or T4 in the blood.
• Elevated levels characterize hyperthyroidism and decreased amounts in hypothyroidism.

Sonographic Evaluation of the Thyroid

• Place the patient in a supine position with a pillow under both shoulders to provide moderate hyperextension of the neck.
• A high-frequency (7.5- to 15-MHz) linear-array transducer is used.

Pathology

• Nodular thyroid disease includes nodular hyperplasia, multinodular goiter, and adenomatous hyperplasia.

Hyperplasia and Goiter

• Nodular hyperplasia, multinodular goiter, and adenomatous hyperplasia are terms used to describe goiter.
• Goiter is the most common thyroid abnormality; it is caused by iodine deficiency.

Goiter

• Goiter is the enlargement of the thyroid gland.
• Enlargement results from compensatory hypertrophy and hyperplasia of follicular epithelium due to deranged hormone secretion.
• It can become very large, compressing the esophagus, interfering with swallowing, or causing pressure on the trachea.
• Other causes of goiter include Graves' disease, thyroiditis, neoplasm, or cyst.
• Toxic goiter is a hyperthyroid condition from thyroid gland hyperactivity.
• Nontoxic (simple) goiter is diffuse thyroid enlargement not resulting from neoplasm or inflammation; not initially associated with hypo- or hyperthyroidism.
• Sonographic findings: most hyperplastic or adenomatous nodules isoechoic compared to normal thyroid tissue; the gland may become hyperechoic; focal scarring and ischemia, as well as necrosis and cyst formation; fibrosis or calcifications occur within the gland.

Cyst

• A cyst in the thyroid gland is cystic degeneration of follicular adenoma.
• Sonographic findings involve degenerative changes corresponding to appearance. Purely anechoic areas suggest serous, colloid fluid, echogenic fluid, or moving fluid; fluid levels may correspond to hemorrhage. 20% of solitary nodules are cystic

Adenoma

• A benign thyroid neoplasm characterized by complete fibrous encapsulation.
• It is characterized by compression of adjacent tissue and fibrous encapsulation.
• Adenomas are homogeneous with variable size, usually solitary with hemorrhage or necrosis areas.
• Adenoma sonographic findings: anechoic to completely hyperechoic commonly with peripheral halo.
• The halo, or thin echolucent rim surrounding lesion, is edema of compressed normal thyroid tissue or capsule of adenoma. Hyperfunction can exhibit increased blood flow patterns on Doppler along peripheral border.

Malignant Lesions

• Carcinoma of the thyroid is rare.
• A solitary nodule may indicate malignancy in a small percentage of cases; the risk declines with multiple nodules.
• A solitary thyroid nodule with ipsilateral cervical adenopathy suggests malignancy.
• Neoplasms can be any single or multiple size, solid, partially cystic, or largely cystic in appearance.
• They are usually hypoechoic relative to normal thyroid and calcifications are present in 50% to 80% of all types of thyroid carcinoma; increased vascularity may be present.

Papillary Carcinoma

• This the most common of thyroid malignancies.
• It affects females more often than males.
• Round, laminated calcifications are seen in 25% of cases.
• The major route of spread is through lymphatics to nearby cervical lymph nodes.
• Approximately 20% of patients with papillary thyroid cancer have metastatic cervical adenopathy.
• Sonographic findings: hypoechogenicity (90% of cases), microcalcifications with or without acoustic shadowing; hypervascularity (90% of cases), and cervical lymph node metastasis (~20% of cases).

Follicular Carcinoma

• Minimally invasive type is well encapsulated; histologically, focal invasion of capsular blood vessels permits differentiation from follicular adenoma.
• Widely invasive type not encapsulated; however, there is invasion of blood vessels and adjacent thyroid tissue.
• Sonographic Findings: irregular margins with thick irregular halo and nodular enlargement with tortuous internal blood vessels.

Medullary Carcinoma

• Accounts for 5% of thyroid cancers.
• It's often familial (20%) and an essential component of multiple endocrine neoplasia (MEN) Type II syndromes.
• In familial cases, it is multi-center and/or bilateral.
• There's a high incidence of metastatic involvement of lymph nodes.
• Lesions appear similar to that of papillary carcinoma as hypoechoic mass, there are often calcium deposits. Sonography highly sensitive in detecting metastatic lymphadenopathy.

Anaplastic Carcinoma

• Anaplastic means undifferentiated; rare; accounts for less than 2% of thyroid cancers.
• It usually occurs after age 50 , presents as hard, fixed mass with rapid growth, and growth that is locally invasive in surrounding neck structures.
• Sonographic Findings: hypoechoic mass, with invasion of surrounding muscles and vessels of neck.

Elastography

• Thyroid elastography (TE) may differentiate TN malignancy .

Lymphoma

• Lymphoma in thyroid is primarily non-Hodgkin's type.
• It affects older women and accounts for 4% of all thyroid malignancies.
• Clinically the patient will have a rapidly growing mass in the neck area.
• The patient may have preexisting chronic lymphocytic thyroiditis (Hashimoto's disease) with subclinical or overt hypothyroidism.
• Nonvascular hypoechoic and lobulated mass is usually present; there may be large areas of cystic necrosis within tumor or encasement of adjacent neck vessels. The adjacent thyroid parenchyma may be heterogeneous secondary to associated chronic thyroiditis.

Thyroiditis

• Types include Acute suppurative thyroiditis, Subacute granulomatous thyroiditis (de Quervain's disease), and Chronic lymphocytic thyroiditis (Hashimoto's disease).

Subacute (de Quervain's) Thyroiditis

• It is caused by a viral infection of thyroid.
• Onset is gradual or abrupt with severe pain.
• May cause transient hyperthyroidism; swelling and pain subside over weeks or months; gland functions normally. The gland appears enlarged and hypoechoic with normal or decreased vascularity.

Hashimoto's Thyroiditis

• The most common form of thyroiditis
• It is characterized by destructive autoimmune disorder, which leads to chronic inflammation of the thyroid.
• It presents as a painless, diffusely enlarged gland in a young or middle-aged female.
• The entire gland is involved with inflammatory reaction; enlargement is not necessarily symmetric.
• Sonographic Findings include diffuse coarsened parenchymal texture slightly more hypoechoic than normal thyroid.
• Homogeneous enlargement initially occurs with nodularity; as disease progresses, gland shows inhomogeneous enlargement (micronodulation). Color Doppler shows normal to decreased flow velocity, occasionally "thyroid inferno" pattern seen when hypothyroidism develops.

Graves' Disease

• Occurs more frequently in women over 30 years of age, related to autoimmune disorder, and characterized by thyrotoxicosis.
• It is is the most frequent cause of hyperthyroidism, and characterized by hypermetabolism, diffuse toxic goiter, exophthalmos, and cutaneous manifestations.
• With hyperthyroidism, there is diffuse hyperplastic goiter, and a clinically thyroid gland that's diffusely homogeneous and enlarged. Sonographically, it is more inhomogeneous than in diffuse goiter, and younger patients may be hypoechoic. Overactivity is manifested with increased vascularity on color Doppler, leading to term "thyroid inferno," and the Spectral Doppler will show velocities over 70 cm/sec.

Parathyroid Gland: Anatomy

• Parathyroid glands are normally located on the posterior medial surface of the thyroid gland.
• There are usually 4, but some may have 3 or 5, and can be found in ectopic places, such as in neck and mediastinum.
• Paired configuration is two posterior to each superior pole of thyroid and two lie posterior to inferior pole. The glands are flat and disc-shaped.
• Echo texture is similar to that of overlying thyroid gland. Normal-size glands measure less than 4 mm and are not generally seen with sonography. Enlarged glands are generally larger than 5 mm and have decreased echo texture and elongated masses between posterior longus colli and anterior thyroid lobe.

Parathyroid Physiology and Laboratory Data

• The parathyroid glands act as calcium-sensing organs and produce parathyroid hormone (PTH).
• They monitor serum calcium feedback mechanism.
• The stimulus to PTH secretion is a decrease in the level of blood calcium.
• When there is decreased serum level, there is increased PTH secretion to stimulate serum calcium to increases.
• When increased serum level occurs, parathyroid activity decreases, and the PTH acts on bone, kidney, and intestine to enhance calcium absorption.
• Unexplained hypercalcemia detected on routine blood chemistry screening is most common referral for parathyroid sonography. Symptomatic renal stones, ulcers, and bone pain are also indications of hyperthyroidism.

Sonographic Evaluation of the Parathyroid Gland

• High-resolution (7.5- to 15-MHz) linear transducer is used.
• Place patients in supine position with neck slightly hyperextended.
• Scan the upper neck (jaw to sternal notch), in transverse and longitudinal planes of thyroid/parathyroid area.
• Ask patient to swallow to elevate thyroid gland during real-time scanning.
• Normal parathyroid glands are oval or bean-shaped -5 x 3 x 1 mm.

Sonographic Evaluation of the Parathyroid Gland Structures

• Longus colli muscle: appears as discrete area posterior to thyroid.
• Longitudinal sections: linear appearance of muscle evident in this plane.
• Minor neurovascular bundle may also be a source of confusion - longitudinal scans can often eliminate confusion by identifying bundle's tubular appearance.

Pathology of the Parathyroid Gland - Primary Hyperparathyroidism

• It is a state of increased function of parathyroid glands.
• Women have primary hyperparathyroidism 2 to 3 times more frequently than men; particularly common after menopause.
• It is characterized by hypercalcemia, hypercalciuria, and low serum levels of phosphate (hypophosphatasia).
• Most patients are asymptomatic at time of diagnosis with no signs of nephrolithiasis or osteopenia.
• Primary hyperparathyroidism occurs when increased amounts of PTH produced by adenoma, primary hyperplasia, or (rarely) carcinoma located in parathyroid gland.

Primary Hyperplasia

• Approximately 10% patients with hyperparathyroidism have parathyroid hyperplasia.
• It is defined as hyperfunction of all parathyroid glands with no apparent cause.
• Only one gland may significantly enlarge, with remaining glands only mildly affected, or all glands may be enlarged (>1 cm).
• Hyperplasia usually involves all glands.
• Multiple adenomas may involve two or three of the glands.
• When the as glands become inconsistently enlarged, becomes more difficult to separate lesions with sonography.
• Correct diagnosis of parathyroid enlargement requires recognition of normal cervical structures (veins, arteries, esophagus. muscles), which can simulate adenomas and give falsly positive results.

Adenoma (Parathyroid gland)

• The most common cause of primary hyperparathyroidism is Adenoma
• Occurs in 80% of cases
• A solitary adenoma involves any one of four glands with equal frequency.
• The most common shape of parathyroid adenoma is oval.
• The As adenoma grows in size, it dissects between longitudinal tissue planes of neck to assume oblong shape, tubelike, or bilobar shape.
• Texture of adenoma is homogeneously solid; occasionally calcifications seen.
• Hypoechoic solid mass smaller than 3 cm in size, with larger masses measuring 5 cms
• Superior parathyroid adenomas are located near the posterior portion of mid thyroid
• Inferior adenomas are more variable, can be near the caudal tips of the distal pole.
• The masses are encapsulated with well defined borders
• Differentiation between adenomas and hyperplasia very difficult by histologic measures
• A hyper vascular pattern or peripheral vascular arc may be found on color Doppler
• It is used to avoid differentiating hyperplastic regional lymph nodes, which are hyper vascular with hilar flow.

Parathyroid Carcinoma

• Histologic differentiation of adenoma and carcinoma is very difficult.
• Metastases must be present for cancer to be determined; Metastases to regional nodes or distant organs, capsular invasion, or local recurrence.
• Cancers of parathyroid glands are generally small, irregular, and firm masses.
• A mass that binds to bordering structures indicates malignancy.
• By way of Sonography, we view that carcinoma is generally larger than adenoma
• It displays a malignant contour, as well as complex internal architecture and cystic architectural component.

Secondary Hyperparathyroidism

• Chronic hypocalcemia caused by renal failure, vitamin D deficiency (rickets), or malabsorption syndromes
• Abnormalities induce PTH secretion, which leads to secondary hyperparathyroidism.
• Hyperfunction of parathyroids is apparently a compensatory reaction
• Renal insufficiency and intestinal malabsorption cause hypocalcemia
• This leads to PTH stimulation

Miscellaneous Neck Masses - Developmental Cysts - Thyroglossal Duct Cysts

• These are congenital anomalies that appear in the midline of neck anterior to trachea.
• Oval or spherical masses are rarely larger than 2 or 3 cm.
• They May persist due to the remaining tubular mass from thyroid development that normally persists between tongue base and hyoid bone
• They are narrow and hollow, attaching the thyroid lobes and attaching to the pharynx floor, they naturally atrophy in adults.
• Failure to atropy creates potential for cystic masses to form anywhere along it.

Branchial Cleft Cysts

• Cystic formations usually located lateral to thyroid gland
• During embryonic development the branchial cleft will extend from the pharyngeal cavity that opens close to the ear
• Diverticulum extends either laterally from pharynx or medially from neck
• Lesions may be solid, with generally a low echogenicity

Abscess

• Primarily fluid-filled to completely echogenic
• Most often shows a low level of echogenicity and highly irregular walls
• Chronic abscesses are challenging to discern due to them naturally blending in
• They need Localization for needle procedures with examination before and after procedure.

Adenopathy

• Normal lymph node oval in shape with homogeneous texture with central core echo complex
• The more rounded the node, the more likely node a possibility exists that it is malignant .
• Low-level echogenicity characterized by classic lymph node shape with close margins indicates enargement
• A node at times can be displayed with echo - free interior
• High inflamed processes will display cyst like properties
• The nature needs to be differentiated with biopsy measures undertaken