Thyroid and Parathyroid Glands

THYROID GLAND

• The thyroid is the largest purely endocrine gland in the body.
• It is a highly vascular, brownish-red, shield-shaped gland situated in the anterior neck.

Functions

• It secretes triiodothyronine (T3), thyroxine (T4), and calcitonin.
• Thyroid hormones regulate the basal metabolic rate and energy expenditure.
• They promote physical growth and development of the nervous system.
• Calcitonin participates in calcium homeostasis by inhibiting osteoclastic bone resorption.

Peculiarities of Thyroid Gland

• Hormone storage: It stores large quantities of thyroid hormones extracellularly within follicular colloid.
• Superficial location: Its position in the anterior neck permits clinical examination by inspection and palpation.
• Iodine requirement: It requires dietary iodine for the synthesis of T3 and T4.
• High vascularity: It receives an exceptionally rich blood supply relative to its size.

Parts

– The thyroid gland is roughly H-shaped and consists of:

1. Right and left lobes: They lie on either side of the larynx and upper trachea.
2. Isthmus: It connects the two lobes across the anterior surface of the trachea.
3. Pyramidal lobe: It is an occasional upward projection, usually arising from the isthmus or adjoining left lobe.

Location and External Features

• The gland occupies the lower anterior and lateral regions of the neck, approximately opposite the C5–T1 vertebral levels.
• Lateral lobes: Each extends superiorly towards the oblique line of the thyroid cartilage and inferiorly to approximately the fifth or sixth tracheal ring.
• Isthmus: It usually overlies the second to fourth tracheal rings.

Weight and Dimension

• Weight: The adult thyroid weighs approximately 20–25 g.
• Lateral lobes: Each measures approximately 5 cm vertically, 3 cm transversely, and 2 cm anteroposteriorly.
• Isthmus: It measures approximately 1.25 cm in height and width, although its dimensions vary.
• The gland is generally larger in females and may enlarge during menstruation and pregnancy.

Pyramidal Lobe

• It may arise from the isthmus or either lateral lobe, usually towards the left.
• Its length and shape are variable.
• It represents a persistent remnant of the thyroglossal duct.

Levator Glandulae Thyroideae

– It is an occasional fibrous or fibromuscular band extending from the pyramidal lobe or isthmus towards the body of the hyoid bone.

Capsules of Thyroid Gland

– The thyroid gland is enclosed by two capsules: True capsule and false capsule.

True Capsule

• It is a thin fibrous covering formed by condensation of the connective tissue stroma of the gland.
• It closely adheres to the thyroid tissue and sends delicate septa into the gland.

False Capsule

• It is derived from the pretracheal layer of deep cervical fascia.
• It surrounds the true capsule and attaches the gland to the adjoining larynx and trachea.
• It is relatively thin posteriorly and becomes thicker along the medial aspect of the lateral lobes.

Suspensory Ligament of Berry

• It is a thickening of the false capsule that anchors the thyroid gland to the cricoid cartilage and upper tracheal rings.
• It maintains the gland’s attachment to the laryngotracheal framework.

• Thyroidectomy: The gland is usually dissected close to its true capsule to minimise injury to the parathyroid glands and their blood supply.
• Recurrent laryngeal nerve injury: The nerve lies close to the ligament of Berry and is particularly vulnerable during thyroid surgery.
• Posterior enlargement: Thyroid enlargement may extend posteriorly and compress neighbouring structures, including the trachea and oesophagus.
• Movement during swallowing: The thyroid gland moves upwards during deglutition because of its attachment to the larynx and trachea through the pretracheal fascia and ligament of Berry.

Parts and Relations of Lobes of Thyroid Gland

– Each thyroid lobe is pyramidal or conical and presents:

1. An apex.
2. A base.
3. Three surfaces: Lateral, medial, and posterolateral.
4. Two borders: Anterior and posterior.

Apex

• The apex is directed upwards and laterally, reaching the oblique line of the thyroid cartilage.
• Laterally: It is related to sternothyroid.
• Medially: It is related to the inferior constrictor of pharynx.

Base

• The base forms the lower end of the lobe and extends approximately to the fifth or sixth tracheal ring.
• It is closely related to the inferior thyroid artery and recurrent laryngeal nerve.

Lateral or superficial surface

– It is convex and covered by the following muscles:

1. Anterior border of sternocleidomastoid.
2. Sternohyoid.
3. Superior belly of omohyoid.
4. Sternothyroid.

Medial surface

– It is related to the following structures:

1. Two tubes: Trachea and oesophagus.
2. Two muscles: Inferior constrictor of pharynx and cricothyroid.
3. Two cartilages: Thyroid and cricoid cartilages.
4. Two nerves: External laryngeal and recurrent laryngeal nerves.

Posterolateral surface

– It is related to the carotid sheath and overlaps the common carotid artery.

Borders of thyroid lobe

Anterior border

• It is thin and separates the lateral and medial surfaces.
• It is related to the anterior branch of the superior thyroid artery.

Posterior border

• It is thick, rounded, and separates the medial and posterolateral surfaces.
• Its relations include:
  • Inferior thyroid artery in its lower part.
  • Anastomosis between the superior and inferior thyroid arteries.
  • Parathyroid glands.
  • Thoracic duct near the lower part of the left lobe.

Parts and Relations of Isthmus of Thyroid Gland

• The isthmus is a flat band of thyroid tissue connecting the right and left lobes.
• It presents two surfaces, anterior and posterior, and two borders, superior and inferior.

Anterior Surface

– It is related to:

1. Sternohyoid.
2. Sternothyroid.
3. Anterior jugular venous connections.

Posterior Surface

– It usually overlies the second to fourth tracheal rings.

H5: Superior Border

– It is related to an arterial anastomosis between the anterior branches of the right and left superior thyroid arteries.

Inferior Border

• The inferior thyroid veins emerge below the isthmus and form a venous plexus anterior to the trachea.
• The thyroidea ima artery, when present, approaches the gland or isthmus from below.

Blood Supply, Lymphatic Drainage and Innervation

Arterial supply

– The thyroid gland receives blood from the following arteries:

1. Superior thyroid artery: It arises from the external carotid artery.
2. Inferior thyroid artery: It arises from the thyrocervical trunk of the subclavian artery.
3. Thyroidea ima artery: It is an occasional artery arising commonly from the brachiocephalic trunk or arch of aorta.
4. Accessory thyroid arteries: Small branches may arise from neighbouring tracheal and oesophageal arteries.

Superior Thyroid Artery

• Origin: It usually arises from the external carotid artery near the carotid bifurcation.
• Course: It descends towards the superior pole of the thyroid gland, accompanied initially by the external branch of the superior laryngeal nerve.
• It passes deep to the infrahyoid muscles and divides into anterior and posterior glandular branches.

Branches

• Anterior branch: It descends along the anterior border of the lobe and anastomoses with its counterpart near the superior border of the isthmus.
• Posterior branch: It runs along the posterior aspect of the lobe and anastomoses with branches of the inferior thyroid artery.

Inferior Thyroid Artery

• Origin: It arises from the thyrocervical trunk of the first part of the subclavian artery.
• Course: It ascends anterior to scalenus anterior and turns medially behind the carotid sheath towards the thyroid gland.
• On the left side, the thoracic duct may cross the artery near its origin or medial course.
• Near the lower pole of the gland, it is closely related to the recurrent laryngeal nerve.
• It divides into several glandular branches that pierce the pretracheal fascia.
• Distribution: It supplies much of the lower and posterior parts of the thyroid gland and contributes to the blood supply of the parathyroid glands.
• Its ascending glandular branches anastomose with branches of the superior thyroid artery.

Venous drainage

– The thyroid gland is drained by three principal groups of veins:

1. Superior thyroid vein: It emerges from the superior pole and drains into the internal jugular vein.
2. Middle thyroid vein: It is a short, wide vein extending from the lateral surface of the thyroid lobe to the internal jugular vein.
3. Inferior thyroid veins: They arise from a venous plexus anterior to the trachea and drain mainly into the brachiocephalic veins.
4. Fourth thyroid vein (Kocher’s vein): It is an occasional vein arising between the middle and inferior thyroid veins and draining into the internal jugular vein.

Lymphatic drainage

– Lymph from the thyroid gland drains into:

1. Prelaryngeal lymph nodes.
2. Upper deep cervical lymph nodes.
3. Pretracheal and paratracheal lymph nodes.
4. Lower deep cervical lymph nodes.
5. Superior mediastinal (brachiocephalic) lymph nodes.

Nerve supply

• The thyroid gland receives sympathetic vasomotor fibres from the superior, middle, and inferior cervical sympathetic ganglia.
• The secretion of thyroid hormones is regulated mainly by thyroid-stimulating hormone (TSH) rather than direct autonomic stimulation.

Histology of Thyroid Gland

• The thyroid gland is enclosed by a thin connective tissue capsule that sends septa into the gland, dividing it into lobules.
• It consists of numerous follicles surrounded by a rich network of capillaries.

Thyroid Follicles

• Each follicle is lined by simple epithelium, usually cuboidal, containing rounded nuclei.
• The epithelial height varies from flattened to columnar according to functional activity.
• The follicular lumen contains homogeneous, eosinophilic colloid, composed mainly of thyroglobulin.

Parafollicular Cells (C Cells)

• They are scattered within the follicular epithelium or between the follicles.
• They secrete calcitonin, which inhibits osteoclastic activity and helps lower blood calcium levels.

Functional Activity of Follicular Cells

• Inactive follicles: They have flattened or low cuboidal epithelial cells and contain abundant colloid.
• Active follicles: They are lined by cuboidal or columnar cells and often contain less colloid.
• Scalloping of colloid: Resorption vacuoles along the colloid margins indicate increased follicular activity. Prominent scalloping is characteristic of Graves’ disease.

Development of Thyroid Gland

• The thyroid is one of the earliest endocrine organs to begin development in the embryo.
• It develops from two embryological components:
  1. Median thyroid primordium: It forms the follicular cells.
  2. Ultimobranchial bodies: They contribute the parafollicular (C) cells.

Development of Follicular Cells

• Around the 24th day, a median endodermal thickening develops in the floor of the primitive pharynx, immediately posterior to the tuberculum impar.
• This thickening forms the thyroid diverticulum, which descends in the neck while remaining temporarily connected to the tongue through the thyroglossal duct.
• The proximal opening of this duct persists as the foramen caecum on the dorsum of the tongue.
• The developing thyroid reaches its normal position anterior to the trachea, while the thyroglossal duct usually disappears.
• Follicular differentiation occurs subsequently, and thyroid hormone synthesis begins around the 12th week of intrauterine life.

Development of Parafollicular Cells

• The ultimobranchial bodies develop from the caudal pharyngeal pouch complex and become incorporated into the thyroid gland.
• They contribute the calcitonin-secreting parafollicular cells.
• Current developmental evidence supports an endodermal origin of mammalian C cells, although older textbooks describe them as neural crest derivatives.

Developmental Anomalies

1. Agenesis or Hypoplasia

– One thyroid lobe or the isthmus may be absent or incompletely developed.

2. Abnormal Location

• Failure of normal thyroid descent may result in thyroid tissue at an unusual location along the thyroglossal duct pathway.
• Common locations include:
  • Lingual thyroid.
  • Intralingual thyroid.
  • Suprahyoid thyroid.
  • Infrahyoid thyroid.

3. Ectopic Thyroid Tissue

• Ectopic thyroid tissue may occur along the normal developmental descent pathway, particularly at the base of the tongue.
• Rare ectopic thyroid tissue has also been reported in the larynx, trachea, oesophagus, and mediastinum.
• Thyroid tissue may occur in ovarian teratomas, where it is termed struma ovarii.

Thyroglossal cyst and fistula

• The thyroid gland develops from a median endodermal primordium that descends through the neck along the thyroglossal duct.
• Normally, the thyroglossal duct disappears during development.
• Persistence of its remnants may produce a thyroglossal cyst or fistula anywhere along the developmental pathway.

Thyroglossal Cyst

• It is a congenital cystic swelling, usually located in the midline of the neck near the hyoid bone.
• It typically moves upwards during swallowing and protrusion of the tongue.
• Infection or rupture of the cyst may produce a draining sinus or fistula opening onto the skin.
• Rarely, carcinoma may develop within a thyroglossal duct cyst.

PARATHYROID GLANDS

• The parathyroid glands are small endocrine glands situated near the posterior aspect of the thyroid gland.
• Usually, four glands are present, arranged as two superior and two inferior parathyroid glands.
• They maintain calcium homeostasis through the secretion of parathyroid hormone (PTH).

Morphology

• Appearance: They are small, yellowish-brown, oval or flattened glands.
• Length: Approximately 6 mm.
• Width: Approximately 3–4 mm.
• Thickness: Approximately 1–2 mm.
• Weight: Each gland weighs approximately 30–50 mg.

Functions

• The chief cells secrete parathyroid hormone (PTH), which increases blood calcium levels.
• PTH acts through the following mechanisms:
  • Bone: It promotes bone resorption indirectly by stimulating osteoclast activity through osteoblast-lineage cells.
  • Kidneys: It increases calcium reabsorption and reduces urinary calcium loss.
  • Vitamin D activation: It stimulates the formation of active vitamin D (calcitriol) in the kidneys.
  • Intestine: It indirectly increases calcium absorption through calcitriol.
  • Phosphate excretion: It reduces renal phosphate reabsorption, increasing phosphate excretion.

Note: PTH increases blood calcium, whereas calcitonin tends to reduce it by inhibiting osteoclastic activity.

Location Of Parathyroid Glands

Superior Parathyroid Glands

• They occupy a relatively constant position near the middle of the posterior border of each thyroid lobe.
• They usually lie posterior (dorsal) to the recurrent laryngeal nerve.

Inferior Parathyroid Glands

• Their position is more variable because of their embryological descent with the thymus.
• They may be situated:
  • Near the lower pole of the thyroid gland, beneath its fascial covering.
  • Behind or below the lower pole, outside the thyroid capsule.
  • Occasionally within the thymus or adjoining cervical tissues.
• They usually lie anterior (ventral) to the recurrent laryngeal nerve.

Clinical Note: The parathyroid glands are closely related to the posterior thyroid capsule and branches of the inferior thyroid artery. These relations help identify and preserve them during thyroidectomy.

Blood supply And Lymphatic Drainage

Arterial Supply

• The parathyroid glands receive a rich arterial supply, mainly from branches of the inferior thyroid artery.
• Superior parathyroid glands: They are usually supplied by the inferior thyroid artery, sometimes with contributions from the superior thyroid artery.
• Inferior parathyroid glands: They receive branches predominantly from the inferior thyroid artery.

Venous Drainage

– Venous blood drains into the thyroid venous plexus and adjoining thyroid veins.

Lymphatic Drainage

– Lymphatics drain into the deep cervical and paratracheal lymph nodes, following the drainage pathways of the thyroid gland and thymus.

Innervation

• The parathyroid glands receive autonomic fibres associated with cervical sympathetic plexuses.
• Their hormonal secretion is regulated primarily by the ionised calcium concentration in blood, rather than by autonomic nerves.
• Low blood calcium stimulates PTH secretion, whereas high blood calcium suppresses it.

Histology

• The parathyroid gland is enclosed by a thin connective tissue capsule.
• Its parenchyma consists of cords, clusters, and sheets of endocrine cells separated by numerous capillaries.
• Two principal cell types are present: Chief cells and oxyphil cells.

Chief Cells

• They are small, numerous, polygonal cells with centrally placed round nuclei and relatively pale cytoplasm.
• They are responsible for the synthesis and secretion of parathyroid hormone.

Oxyphil Cells

• They are larger and fewer than chief cells.
• They contain abundant eosinophilic cytoplasm and small, dark-staining nuclei.
• They usually become prominent after puberty and increase in number with age.
• Their precise physiological role remains uncertain, although they may retain some capacity for PTH production.

Development

– The parathyroid glands develop from the endoderm of the third and fourth pharyngeal pouches.

Superior Parathyroid Glands

• They develop from the dorsal part of the fourth pharyngeal pouch.
• They separate from the pharyngeal wall and descend a relatively short distance to reach the posterior aspect of the thyroid gland.
• They are also called parathyroid IV.

Inferior Parathyroid Glands

• They arise from the dorsal part of the third pharyngeal pouch.
• They descend caudally along with the developing thymus.
• They subsequently separate from the thymus and usually settle near the lower poles of the thyroid gland.
• They are also called parathyroid III.

Note: The inferior parathyroid glands have a more variable location because they migrate farther than the superior glands.