Thyroid and Parathyroid Glands

Chapter 22: Thyroid and Parathyroid Glands

Anatomy of the Thyroid Gland

  • Located in the anteroinferior neck at the level of thyroid cartilage.

  • Consists of:

    • Right and Left Lobes.

    • Connected by the isthmus.

Anatomical Relationships

  • Straddles the trachea anteriorly.

  • Bounded laterally by:

    • Carotid arteries.

    • Jugular veins.

  • Pyramidal Lobe:

    • Arises from the isthmus, sometimes seen in the thyroid anatomy.

Blood Supply and Drainage

  • Blood Supply:

    • Superior Thyroid Artery: Arises from external carotids, descends to upper poles.

    • Inferior Thyroid Artery: Arises from thyrocervical trunk of subclavian artery, ascends to lower poles.

  • Blood Flow Velocities:

    • Major thyroid arteries: peak systolic velocities of 20 to 40 cm/sec.

    • Intraparenchymal arteries: 15 to 30 cm/sec.

  • Venous Drainage:

    • Corresponding veins drain into internal jugular veins.

Size Specifications

  • Thyroid Lobes: Should normally be equal in size.

    • Newborn dimensions: 18 to 20 x 8 to 9 mm.

    • Normal adult thyroid dimensions: 40 to 60 x 20 x 13 to 18 mm.

    • Isthmus: 4 to 6 mm in anteroposterior (AP) diameter.

Volume Measurements

  • Thyroid volume is calculated using an ellipsoid formula with a correction factor:

    • Formula: V=length×width×thickness×0.52V = \text{length} \times \text{width} \times \text{thickness} \times 0.52

  • Normal mean thyroid volume: 18.6 ± 4.5 ml.

  • Notable Differences:

    • Volume in males is slightly larger than in females.

Relational Anatomy

Anterior Structures

  • Anterior surface includes strap muscles:

    • Sternothyroid

    • Omohyoid

    • Sternohyoid

    • Sternocleidomastoid

  • Muscular Arrangement:

    • Sternohyoid and Omohyoid: Thin, hypoechoic bands anterior to gland.

    • Sternocleidomastoid muscle: Larger oval band that lies anterior and lateral to gland.

Posterior Anatomy

  • Posterolateral anatomy includes:

    • Common carotid artery

    • Internal jugular vein

    • Vagus nerve

  • Longus colli muscle: Located posterior and lateral to lobes; identifiable as a hypoechoic triangular structure adjacent to cervical vertebrae.

Medial Structures

  • Medial anatomy includes:

    • Larynx

    • Trachea

    • Inferior constrictor of pharynx

    • Esophagus

  • Esophagus: Primarily midline but may be found to the left of trachea, identifiable by target appearance in transverse view and peristaltic motion when the patient swallows.

  • Posterior border of each lobe relates to superior and inferior parathyroid glands and the anastomosis between the superior and inferior thyroid arteries.

Thyroid Physiology and Laboratory Data

Function and Hormone Secretion

  • The thyroid gland maintains normal body metabolism, growth, and development by synthesis, storage, and secretion of thyroid hormones.

  • Iodine Metabolism: Fundamental mechanism for producing thyroid hormones.

  • Hormones Produced:

    • Triiodothyronine (T3)

    • Thyroxine (T4)

  • Thyroid gland traps iodine from blood, which through a series of chemical reactions produces T3 and T4.

  • Upon demand, thyroid hormones are released into bloodstream regulated by thyrotropin, or thyroid-stimulating hormone (TSH) from the pituitary gland.

Calcitonin Function

  • Calcitonin reduces blood calcium concentrations by acting on bones to inhibit breakdown, thus contributing to calcium homeostasis in the bloodstream.

Euthyroid State

  • The state where the thyroid produces the correct amount of hormones is referred to as being euthyroid.

Hypothyroidism

  • Definition: Undersecretion of thyroid hormones.

  • Causes:

    • Low iodine intake (can lead to goiter)

    • Inability of the thyroid to produce sufficient hormone

    • Pituitary gland dysfunction affecting thyroid regulation

  • Clinical Signs:

    • Myxedema

    • Weight gain

    • Hair loss

    • Increased subcutaneous tissue, particularly around the eyes

    • Lethargy

    • Intellectual and motor slowing

    • Cold intolerance

    • Constipation

    • Deep husky voice.

Hyperthyroidism

  • Definition: Oversecretion of thyroid hormones.

  • Occurs generally when the whole gland is overactive or localized neoplasms (e.g., adenomas) cause hormone overproduction.

  • Clinical Signs:

    • Dramatic weight loss

    • Increased appetite

    • Heightened nervous energy

    • Tremors

    • Excessive sweating

    • Heat intolerance

    • Palpitations

    • Potential for exophthalmos (protruding eyes).

Tests of Thyroid Function

  • Nuclear Medicine is utilized to determine thyroid function, including:

    • Iodine uptake scans

    • Thyroid scans.

  • Laboratory Tests:

    • Measure levels of T3 and T4 in blood.

    • T3 and T4 levels are elevated in hyperthyroidism but reduced in hypothyroidism.

Sonographic Evaluation of the Thyroid

  • It's essential for the patient to be in a supine position with a pillow under both shoulders to provide moderate hyperextension of the neck.

  • Equipment used includes:

    • High-frequency (7.5- to 15-MHz) linear-array transducer.

  • Sonographic descriptions should include specifics of measurements:

    • Right Thyroid:

    • Length: 3.74 cm

    • Height: 1.25 cm

    • Isthmus Width:

    • 0.24 cm

Pathology of the Thyroid Gland

Nodular Thyroid Disease

  • Terminology includes:

    • Nodular hyperplasia

    • Multinodular goiter

    • Adenomatous hyperplasia

  • Goiter is the most common thyroid abnormality and is mainly caused by iodine deficiency.

Types of Goiter

  1. Toxic Goiter: Hyperthyroid condition resulting from hyperactivity of the gland.

  2. Nontoxic (Simple) Goiter: Occurs as diffuse thyroid enlargement that is not caused by neoplasm or inflammation—initially not linked to hypo- or hyperthyroidism.

  3. Sonographic Findings concluding descriptions of goiters related to their echo patterns and presence of abnormalities (e.g., calcifications).

Cysts in the Thyroid Gland

  • Cyst Definition: Cystic degeneration of a follicular adenoma.

  • Sonographic Findings include:

    • Anechoic areas from serous or colloid fluid.

    • Degenerative changes lead to varying sonographic appearances, including fluid levels indicative of hemorrhagic events.

Benign Neoplasms (Adenoma)

  • Benign thyroid neoplasm characterized by:

    • Complete fibrous encapsulation.

    • Sole lesion localized with areas of hemorrhage or necrosis.

  • Sonographic characteristics include:

    • Variability in echo patterns

    • Increased blood flow patterns may be seen in hyperfunctioning adenomas.

Malignant Lesions of the Thyroid

  • Carcinoma: Rare occurrences with a small risk of single nodules being malignant. Solitary thyroid nodule with cervical adenopathy raises suspicion of malignancy.

  • Common Types:

    • Papillary Carcinoma: Most prevalent cancer found in the thyroid; characterized by:

    • Round, laminated calcifications.

    • Spread through lymphatics.

    • Follicular Carcinoma: Differentiated based on invasion of blood vessels and surrounding tissue.

    • Medullary Carcinoma: Often familial or part of multiple endocrine neoplasia (MEN) syndromes; shows distinct sonographic findings.

    • Anaplastic Carcinoma: Rare, highly aggressive, often presents as hard and fixed with rapid growth.

    • Lymphoma: Primarily non-Hodgkin’s type, characterized by rapid growth in neck and possible correlation with chronic lymphocytic thyroiditis (Hashimoto’s disease).

Types of Thyroiditis

  • Includes:

    • Acute Suppurative Thyroiditis

    • Subacute Granulomatous Thyroiditis (de Quervain’s disease)

    • Chronic Lymphocytic Thyroiditis (Hashimoto’s disease) - most common form; characterized by autoimmune destruction leading to hypothyroidism.

  • Sonographic Findings: Generally show enlarged, heterogeneous parenchyma.

Graves’ Disease

  • An autoimmune disorder, prevalent in women over 30, leading to hyperthyroidism characterized by:

    • Hypermetabolism

    • Diffuse goiter

    • Exophthalmos

Anatomy of Parathyroid Glands

  • Normal locations include the posterior medial surface of thyroid gland.

  • Typically, four glands (possible variation of three to five).

  • Size and Sonographic Appearance:

    • Normal glands (<4 mm) not typically visible. Enlarged glands (>5 mm) present as elongated masses, particularly distinguished from surrounding structures.

Parathyroid Physiology and Laboratory Data

  • Function includes monitoring serum calcium levels and producing parathyroid hormone (PTH).

  • Trigger for PTH Release: Decrease in serum calcium levels.

  • PTH's role involves:

    • Bone, kidney, and intestine interaction to promote calcium absorption.

Pathology of the Parathyroid Gland

Conditions of Hyperparathyroidism

Primary Hyperparathyroidism:

  • Increased function of parathyroid glands, more prevalent in women, especially post-menopausal.

  • Characterized by:

    • Hypercalcemia

    • Low serum phosphate levels (hypophosphatasia).

Primary Hyperplasia:

  • Hyperfunction of all parathyroid glands.

  • Defined by the overall growth of one or more glands without apparent cause.

Parathyroid Adenomas:

  • Most common cause of primary hyperparathyroidism (80% of cases), appearing as discrete oval masses.

Miscellaneous Neck Masses

Types of Developmental Cysts

  • Thyroglossal Duct Cysts: Congenital anomalies that may appear in the midline, usually small and benign.

  • Branchial Cleft Cysts: Typically located laterally; occur during embryonic development with solid components.

Sonographic Findings for Abscesses and Adenopathy

  • Abscesses may appear fluid-filled or echogenic with irregular walls.

  • Normal lymph nodes exhibit a homogeneous elliptical appearance, with variances indicative of malignancy or inflammation.

Conclusion

  • Thyroid and parathyroid glands have essential roles in metabolism and calcium homeostasis, which can be disrupted by various pathologies, necessitating comprehensive evaluation and management strategies in both clinical and sonographic practices.