The Thyroid and Parathyroid Glands

Anatomy and Measurements of the Thyroid Gland

• Location: The thyroid gland is situated in the anteroinferior portion of the neck at the level of the thyroid cartilage.

• Structure: It consists of a Right lobe and a Left lobe, which are connected across the midline by the isthmus.

• Positioning: The gland straddles the trachea anteriorly. It is bounded laterally by the carotid arteries and the internal jugular veins.

• Pyramidal Lobe: A pyramidal lobe sometimes arises from the isthmus and extends superiorly.

• Specific Anatomy Landmarks:     * Superior thyroid artery and vein.     * Middle thyroid vein.     * Inferior thyroid vein.     * Cricoid cartilage.     * Trachea.

• Normal Size Parameters:     * Lobes: Usually equal in size.     * Newborn: Approximately $18\text{ to }20 \times 8\text{ to }9\,\text{mm}$.     * Normal Adult: Dimensions are approximately $40\text{ to }60 \times 20 \times 13\text{ to }18\,\text{mm}$.     * Isthmus: The anteroposterior (AP) diameter ranges from $4\text{ to }6\,\text{mm}$.

• Volume Calculation:     * The common method is based on the ellipsoid formula with a correction factor applied to each lobe: $\text{length} \times \text{width} \times \text{thickness} \times 0.52$.     * Mean Thyroid Volume: Normal mean is $18.6 \pm 4.5\,\text{ml}$.     * Gender Differences: Volume in males is typically slightly larger than in females.

Relational Anatomy of the Thyroid Gland

• Anterior Relations:     * The anterior surface is bordered by the strap muscles: sternothyroid, omohyoid, and sternohyoid.     * The sternocleidomastoid muscle is a larger oval band lying anterior and lateral to the gland.     * Sternohyoid and omohyoid muscles appear as thin, hypoechoic bands on ultrasound.

• Posterior Relations:     * Posterolateral Anatomy: Includes the common carotid artery, internal jugular vein, and the vagus nerve.     * Longus Colli Muscle: Located posterior and lateral to the lobes. It is a hypoechoic triangular structure adjacent to the cervical vertebrae (C5).     * Minor Neurovascular Bundle: Situated posteriorly.

• Medial Relations:     * Includes the larynx, trachea, inferior constrictor of the pharynx, and the esophagus.     * Esophagus: Primarily a midline structure, often found to the left of the trachea. It is identified by a "target" appearance in the transverse plane and visible peristaltic movements when the patient swallows.

• Posterior Border Interactions:     * Related to the superior and inferior parathyroid glands.     * Site of anastomosis between the superior and inferior thyroid arteries.     * Proximity to the recurrent laryngeal nerve.

Blood Supply and Physiology

• Arterial Supply:     * Superior Thyroid Arteries: Two arteries arising from the external carotids that descend to the upper poles of the gland.     * Inferior Thyroid Arteries: Two arteries arising from the thyrocervical trunk of the subclavian artery that ascend to the lower poles.

• Venous Drainage: Corresponding veins drain directly into the internal jugular veins.

• Doppler Peak Systolic Velocities:     * Major Thyroid Arteries: Range from $20\text{ to }40\,\text{cm/sec}$.     * Intraparenchymal Arteries: Range from $15\text{ to }30\,\text{cm/sec}$.

• Thyroid Physiology:     * Function: Maintenance of normal body metabolism, growth, and development through hormone synthesis, storage, and secretion.     * Mechanism: Iodine metabolism. The gland traps iodine from the blood.     * Hormones Produced: Triiodothyronine ($T3$) and Thyroxine ($T4$).     * Regulation: Pituitary gland produces thyrotropin, or thyroid-stimulating hormone ($TSH$), to trigger hormone release into the bloodstream.     * Calcitonin: Helps maintain blood calcium homeostasis by inhibiting bone breakdown to decrease calcium concentration in the blood.     * Euthyroid: A state where the thyroid is producing the correct/normal amount of hormone.

Thyroid Dysfunction and Clinical Manifestations

• Hypothyroidism (Undersecretion):     * Causes: Low iodine intake (goiter), intrinsic gland inability to produce hormones, or pituitary gland dysfunction.     * Clinical Signs: Myxedema, weight gain, hair loss, increased subcutaneous tissue around eyes, lethargy, intellectual and motor slowing, cold intolerance, constipation, and a deep husky voice.

• Hyperthyroidism (Oversecretion):     * Causes: The entire gland being out of control or a localized neoplasm (adenoma) causing overproduction.     * Clinical Signs: Weight loss, increased appetite, high nervous energy, tremors, excessive sweating, heat intolerance, and palpitations.     * Exophthalmos: Protruding eyes, frequently seen in hyperthyroid patients.

• Diagnostic Testing:     * Nuclear Medicine: Iodine uptake scans and thyroid scans determine functional status.     * Laboratory Tests: Measuring blood levels of $T3$ or $T4$. Levels are elevated in hyperthyroidism and decreased in hypothyroidism.

Sonographic Evaluation of the Thyroid

• Patient Positioning: Supine with a pillow under the shoulders to achieve moderate hyperextension of the neck.

• Transducer: High-frequency ($7.5\text{ to }15\,\text{MHz}$) linear-array transducer.

• Imaging Planes:     * Sagittal (Longitudinal): Imaging the right lateral, right mid, and right medial aspects. Measurements for length and height are taken here.     * Transverse: Imaging the upper, mid, and lower aspects of the lobes and the isthmus.

• Landmarks:     * Trachea (Tr): Midline, posterior to the isthmus ($Is$) with posterior shadowing.     * Common Carotid Artery (CCA): Circular, pulsatile structure lateral and adjacent to the gland.     * Internal Jugular Vein (IJV): Oval-shaped, lateral to the CCA.     * Esophagus: Adjacent to the trachea with a hypoechoic rim and echogenic center; peristalsis noted with swallowing.

• Measurement Protocol: Obtain three measurements of each lobe (maximum length, AP, and width) for volume calculations.

Nodular Thyroid Disease and Goiter

• Terminology: Nodular hyperplasia, multinodular goiter, and adenomatous hyperplasia.

• Goiter Definition: Enlargement of the thyroid gland due to compensatory hypertrophy and hyperplasia of follicular epithelium.

• Causes: Most commonly caused by iodine deficiency; other causes include Graves’ disease, thyroiditis, neoplasms, or cysts.

• Classifications:     * Toxic Goiter: A hyperthyroid condition resulting from thyroid hyperactivity.     * Nontoxic (Simple) Goiter: Diffuse enlargement not caused by neoplasm or inflammation; initially not associated with hypo- or hyperthyroidism.

• Sonographic Findings of Goiter:     * Nodules may be poorly circumscribed or well-defined and encapsulated by a thin, peripheral hypoechoic halo.     * Most nodules are isoechoic to normal tissue; may become hyperechoic as the gland enlarges.     * Common features include focal scarring, ischemia, necrosis, cyst formation, fibrosis, and calcifications.     * Multinodular Goiter: Inhomogeneous enlarged mass with increased vascularity.

Benign Lesions: Cysts and Adenomas

• Thyroid Cysts:     * Usually cystic degeneration of a follicular adenoma.     * Incidence: Approximately $20\%$ of solitary nodules are cystic.     * Sonographic Findings: Purely anechoic areas (serous/colloid fluid) or echogenic/moving fluid; fluid levels may indicate hemorrhage. Colloid cysts may show echogenic foci.

• Thyroid Adenoma:     * A benign neoplasm with complete fibrous encapsulation.     * Appearance: Homogeneous, solitary, variable in size. Compresses adjacent tissue.     * Sonographic Findings: Range from anechoic to hyperechoic. Often features a "halo" (thin echolucent rim) caused by the capsule or edema of compressed tissue.     * Doppler: Hyperfunctioning adenomas may show increased blood flow on peripheral borders or within the lesion.

Malignant Lesions of the Thyroid

• General Characteristics:     * Carcinoma is rare; solitary nodules have a higher risk of malignancy than multiple nodules.     * Solitary nodules paired with ipsilateral cervical adenopathy are highly suspicious.     * Sonographic Markers: Solid, markedly hypoechoic masses (compared to strap muscles), irregular/microlobulated margins, "taller than wide" shape, and punctate internal microcalcifications (< 2\,\text{mm}).     * Calcifications: Present in $50\% \text{ to } 80\%$ of all thyroid carcinomas.

• Papillary Carcinoma:     * Most common thyroid malignancy; affects females more than males.     * Spread: Major route through lymphatics to cervical lymph nodes ($20\%$ have metastatic cervical adenopathy).     * Sonographic Findings: Hypoechogenicity ($90\%$), hypervascularity ($90\%$), and round laminated calcifications (seen in $25\%$ of cases).

• Follicular Carcinoma:     * Minimally Invasive: Well-encapsulated; focal invasion of capsular blood vessels.     * Widely Invasive: Not encapsulated; invades blood vessels and adjacent tissue.     * Sonographic Findings: Irregular margins, thick irregular halo, nodular enlargement, and tortuous internal blood vessels.

• Medullary Carcinoma:     * Accounts for $5\%$ of thyroid cancers.     * Familial Link: $20\%$ are familial and part of multiple endocrine neoplasia (MEN) type II syndromes.     * Sonographic Findings: Appears similar to papillary carcinoma (hypoechoic mass with calcium deposits).

• Anaplastic Carcinoma:     * Undifferentiated and rare (< 2\%); usually occurs after age 50.     * Presentation: Hard, fixed mass with rapid, locally invasive growth into neck structures.     * Sonographic Findings: Hypoechoic mass invading surrounding muscles and vessels.

• Lymphoma:     * Primarily non-Hodgkin’s type; affects older women ($4\%$ of malignancies).     * Linked to preexisting Hashimoto’s disease.     * Sonographic Findings: Large, nonvascular, hypoechoic, and lobulated mass; may have areas of cystic necrosis.

• Elastography:     * Used to assess tissue stiffness.     * Soft areas appear blue; hard areas appear red.

Thyroiditis and Graves' Disease

• Types of Thyroiditis:     * Acute suppurative thyroiditis.     * Subacute Granulomatous Thyroiditis (de Quervain’s Disease): Caused by viral infection. Features abrupt onset of pain and transient hyperthyroidism. Sonographically enlarged and hypoechoic with decreased vascularity.     * Chronic Lymphocytic Thyroiditis (Hashimoto’s Disease): Most common form; destructive autoimmune disorder leading to chronic inflammation.

• Hashimoto’s Characteristics:     * Painless, diffuse enlargement, common in young/middle-aged females.     * Sonographic Findings: Diffuse coarsened texture, homogeneous enlargement initially followed by micronodulation (inhomogeneous enlargement). Normal to decreased flow, but sometimes demonstrates a "thyroid inferno" pattern during hypothyroidism.

• Graves’ Disease:     * Autoimmune disorder; most frequent cause of hyperthyroidism (thyrotoxicosis).     * Findings: Hypermetabolism, diffuse toxic goiter, exophthalmos, and cutaneous manifestations.     * Sonographic Findings: Inhomogeneous echogenicity, diffusely enlarged. Overactivity is marked by increased vascularity known as "thyroid inferno."     * Velocities: Spectral Doppler may show velocities exceeding $70\,\text{cm/sec}$.

Parathyroid Gland Anatomy and Physiology

• Location: Usually on the posterior medial surface of the thyroid gland.

• Number: Typically four (paired), though some individuals have three or five. Two lie posterior to the superior poles, and two lie posterior to the inferior poles.

• Ectopic Locations: Found in the neck and mediastinum.

• Morphology: Flat and disc-shaped.

• Normal Size: $5\,\text{mm} \times 3\,\text{mm} \times 1\,\text{mm}$. Normal glands (< 4\,\text{mm}) are usually not seen with ultrasound.

• Enlarged Glands: (> 5\,\text{mm}) appear as hypoechoic elongated masses between the posterior longus colli and anterior thyroid lobe.

• Physiology:     * Calcium-sensing organs that produce parathyroid hormone ($PTH$).     * Mechanism: Decrease in blood calcium stimulates $PTH$ secretion.     * $PTH$ acts on bone, kidney, and intestine to enhance calcium absorption.     * Hyperactivity leads to hypercalcemia.

• Clinical Indications for Scanning: Unexplained hypercalcemia, symptomatic renal stones, ulcers, and bone pain.

Pathology of the Parathyroid Glands

• Primary Hyperparathyroidism:     * State of increased function; common in women after menopause (2-3 times more frequent than men).     * Characterized by hypercalcemia, hypercalciuria, and low serum phosphate (hypophosphatasia).

• Primary Hyperplasia:     * Hyperfunction of all glands without apparent cause (occurs in $10\%$ of hyperparathyroidism patients).     * All glands or just one may enlarge (> 1 \,\text{cm}).

• Parathyroid Adenoma:     * Most common cause of primary hyperparathyroidism ($80\%$).     * Usually solitary, oval, and solid (hypoechoic). Most are < 3\,\text{cm}.     * Doppler: May show a “peripheral vascular arc.–

• Parathyroid Carcinoma:     * Requires evidence of metastases, capsular invasion, or local recurrence for diagnosis.     * Usually larger than adenomas with a lobular contour and heterogeneous internal architecture.

• Secondary Hyperparathyroidism:     * Compensatory reaction to chronic hypocalcemia caused by renal failure, Vitamin D deficiency (rickets), or malabsorption. All four glands are usually enlarged.

Miscellaneous Neck Masses and Lymphadenopathy

• Thyroglossal Duct Cysts:     * Congenital midline anomalies anterior to the trachea.     * Remnant of the tract between the tongue base and hyoid bone. Usually < 2\text{ to }3\,\text{cm}.

• Branchial Cleft Cysts:     * Cystic formations lateral to the thyroid gland arising from embryonic tracts. May contain low-level internal echoes.

• Abscess:     * Range from fluid-filled to completely echogenic with irregular walls. Chronic versions may have indistinct margins.

• Adenopathy (Lymph Nodes):     * Normal: Oval, homogeneous texture with a central core echo complex (hilum).     * Malignancy: Rounder shape and low-level echogenicity. Well-circumscribed hypoechoic masses generally suggest enlargement.     * Diagnosis: Often requires fine-needle biopsy to differentiate inflammation from neoplasm.