Anatomy, Histology, and Development of the Thyroid Gland

Comprehensive practice flashcards defining key anatomical structures, surgical landmarks, clinical relations, histology, physiology, and embryology of the thyroid gland.

Accessory thyroid

Nodules of normal thyroid tissue found situated near the main thyroid gland.

Ectopic thyroid

Thyroid tissue located in an abnormal position, such as within the posterior triangle of the neck or inside the thorax, resulting from faulty embryological migration.

Pretracheal fascia

The layer of deep cervical fascia that forms the outer false capsule enclosing the thyroid gland, attaching superiorly to the hyoid bone and cricoid cartilage.

True capsule of the thyroid gland

The inner condensation of connective tissue covering the thyroid gland, lying immediately superficial to the peripheral venous plexus.

Ligament of Berry

A suspensory ligament formed by the pretracheal fascia that anchors the thyroid gland to the cricoid cartilage and hyoid bone, causing the gland to elevate and depress during deglutition.

Thyroid surgical cleavage plane

The plane between the inner true capsule and outer false capsule where incisions are made during thyroid surgery to avoid massive venous bleeding from the underlying venous plexus.

Postoperative tension hematoma

A life-threatening complication occurring after thyroid surgery when bleeding collects under tension within a closed pretracheal fascia, directly compressing the tracheal wall and causing acute respiratory choking.

Lateral lobe dimensions (Thyroid gland)

The standard anatomic dimensions of each lateral lobe: length of $5\,\text{cm}$, breadth of $3\,\text{cm}$, and thickness of $2\,\text{cm}$.

Sternothyroid muscle

An infrahyoid strap muscle whose line of attachment to the thyroid lamina physically prevents the upward enlargement or tumor expansion from the apex of the thyroid gland.

External laryngeal nerve

A branch of the vagus nerve closely related behind the superior thyroid artery at the thyroid apex that innervates solely the cricothyroid muscle to tense and lengthen vocal folds for high-pitched sounds.

Superior thyroid artery ligation

The surgical practice of tying off the superior thyroid artery close to the apex of the thyroid gland to avoid injuring the nearby external laryngeal nerve.

Recurrent laryngeal nerve (RLN)

A nerve intimately related to the base of the thyroid gland and the inferior thyroid artery that innervates all intrinsic laryngeal muscles except the cricothyroid.

Distal ligation of the inferior thyroid artery

The ligation of the inferior thyroid artery close to the thyroid capsule rather than near its origin, preserving the arterial blood supply to the parathyroid glands to prevent postoperative hypocalcemia.

Anterolateral surface (Thyroid lobe)

The superficial aspect of each lateral lobe protected by an overlying muscle shield composed of the sternothyroid, sternohyoid, superior belly of the omohyoid, and sternocleidomastoid muscles.

Medial surface relations (Thyroid gland)

The anatomical structures related to the medial surface of each lobe, comprising two tubes (trachea, esophagus), two muscles (cricothyroid, inferior constrictor), and two nerves (external laryngeal, recurrent laryngeal).

Posterolateral surface (Thyroid lobe)

The lateral surface of each thyroid lobe that rests against the carotid sheath (enclosing the common carotid artery, internal jugular vein, and vagus nerve) and the cervical sympathetic chain.

Dyspnea (Thyroid compression)

Shortness of breath or respiratory distress resulting from compression or displacement of the trachea by an enlarged thyroid gland.

Dysphagia (Thyroid compression)

Difficulty in swallowing caused by direct posterior mechanical compression of the cervical esophagus by a goiter.

Horner's syndrome (Thyroid etiology)

A neurological condition manifesting with ptosis, pupil constriction, and facial anhidrosis caused by pathological thyroid mass compression of the cervical sympathetic chain.

Thyroid isthmus

The narrow central band connecting the right and left lateral thyroid lobes, lying anterior to the second, third, and fourth tracheal rings.

Pyramidal lobe

An accessory vertical elongation extending upward from the isthmus toward the hyoid bone, present in a subset of individuals as a remnant of the thyroglossal duct.

Tracheostomy (Standard site)

An operative airway entry made into the trachea below the cricoid cartilage at the level of the second to third tracheal rings, requiring retraction or division of the thyroid isthmus.

Superior belly of the omohyoid muscle

An anatomic landmark used during neck surgery to locate the superior and middle thyroid veins along its inner and outer borders.

Kocher's vein

An accessory middle thyroid vein draining directly from the lateral aspect of the thyroid lobe into the internal jugular vein.

Inferior thyroid veins

Multiple venous tributaries arising from the lower border of the thyroid isthmus that descend into the superior mediastinum to drain into the left brachiocephalic vein.

Thyroid follicles

Approximately 3 million structural and functional units of the thyroid, each composed of a simple epithelial lining resting on a basement membrane enclosing a colloid core.

Follicular epithelium functional states

The variable morphology of follicular cells reflecting synthesis: columnar cells during high secretion, cuboidal cells during moderate activity, and flat/squamous cells when inactive.

Thyrocalcitonin (Calcitonin)

A peptide hormone secreted by parafollicular cells that decreases serum calcium by inhibiting osteoclast bone resorption and enhancing osteoid calcification.

Thyroid peroxidase (TPO)

A key follicular enzyme responsible for oxidizing transported iodide into iodine and catalyzing the iodination of tyrosine residues on thyroglobulin.

Pendrin

An apical iodide-chloride transporter that shuttles iodide from follicular cell cytoplasm across the apical membrane into the colloid lumen.

Goiter

An abnormal enlargement of the thyroid gland, commonly triggered globally by dietary iodine deficiency leading to excessive thyroid-stimulating hormone (TSH) stimulation.

Cretinism

A severe congenital condition caused by untreated fetal or infantile hypothyroidism, clinically recognized by stunted body growth and irreversible intellectual disability.

Myxedema

The adult manifestation of severe hypothyroidism characterized by lethargy, cold intolerance, dry skin, non-pitting edema, weight gain, and mental slowing.

Parafollicular cells (C cells)

Polyhedral neuroendocrine cells with eccentric nuclei and pale cytoplasm situated between follicular cells and their basement membrane, derived embryologically from the ultimobranchial body.

Thyroglossal duct

A median endodermal diverticulum originating from the floor of the primitive pharynx at the foramen cecum that migrates caudally to give rise to the thyroid parenchyma.

Lingual thyroid

A developmental failure where incomplete caudal migration of the thyroglossal duct arrests thyroid development inside the base of the tongue.

Thyroglossal cyst

A midline congenital cervical swelling arising from persistent remnants of the thyroglossal duct that characteristically elevates during deglutition.