Largest endocrine gland. H-shaped. Lies at C5–T1, clasping upper trachea.

Hormones: T3 + T4 (metabolic rate, growth); Calcitonin (↓blood calcium; from parafollicular C cells; opposes PTH)

Special features: Only endocrine gland that is superficial, iodine-dependent, stores hormones extracellularly (as colloid), and is one of the most vascular organs.

Parts and Dimensions

Pyramidal lobe — ~50%; projects upward from isthmus (usually left of midline); represents persistent lower thyroglossal duct; may be connected to hyoid by levator glandulae thyroideae

Capsules

True capsule — thin fibrous; septa divide gland into lobules (20–40 follicles each)

False capsule (pretracheal fascia sheath) — from pretracheal layer of deep cervical fascia; loose sheath; posteriorly forms suspensory ligament of Berry (attaches posteromedial lobe to cricoid + 1st/2nd tracheal rings)

RLN passes posterior/just lateral to ligament of Berry before entering larynx — at risk during ligation near the ligament

Why thyroid moves on swallowing: False capsule derived from pretracheal fascia → attached to thyroid/cricoid/hyoid → these ascend on deglutition → gland moves upward = hallmark clinical sign

Space between capsules: Contains parathyroid glands (4), terminal thyroid artery branches, thyroid vein tributaries → surgical plane during thyroidectomy

Relations

Anterolateral (superficial) surface (superficial→deep): skin, superficial fascia (platysma), investing deep fascia, anterior jugular veins, sternohyoid, omohyoid, sternothyroid (directly applied; attachment to oblique line of thyroid cartilage limits upward expansion → goitres enlarge downward → retrosternal)

Medial surface: cricothyroid muscle + EBSLN (upper pole), thyroid cartilage, cricoid, trachea (1st–6th rings), cricothyroid joint, inferior constrictor, oesophagus (→ left side; left lobe goitre → dysphagia more readily), RLN in tracheo-oesophageal groove (most important surgical relation)

Posterolateral surface: common carotid artery (medial), IJV (lateral), vagus (between), sympathetic trunk, parathyroid glands

Arterial Supply

Superior: ligate at upper pole capsule — protects EBSLN Inferior: ligate laterally — protects RLN and parathyroids Thyroidea ima: identify before tracheostomy/thyroidectomy — unexpected haemorrhage if cut unrecognised

RLN crossing inferior thyroid artery: may pass anterior, posterior, or between branches at posteromedial lobe

Venous Drainage

Middle thyroid vein — no accompanying artery; ligated first to mobilise lobe; tearing = brisk IJV haemorrhage Inferior thyroid veins form thyroid plexus in front of trachea — important in tracheostomy

Lymphatic Drainage

1st echelon (Level VI/VII): Prelaryngeal (Delphian node — sentinel for thyroid Ca), pretracheal, paratracheal nodes

→ Deep cervical nodes (II–IV) → Superior mediastinal nodes (lower pole/isthmus)

Delphian node: enlarged hard midline prelaryngeal swelling = predicts thyroid malignancy

Nerve Supply

• Only vasomotor (sympathetic) fibres — no secretomotor innervation; hormone secretion regulated by TSH
• Sympathetic: superior/middle/inferior cervical ganglia → along thyroid arteries

Nerves at risk in surgery:

Microscopic Structure

Follicles — spherical sacs; wall = single layer of follicular cells (thyrocytes); lumen = colloid (PAS-positive thyroglobulin)

Parafollicular C cells — in follicle wall or interfollicular stroma, never lining lumen; large, pale polygonal; origin = neural crest → ultimobranchial body (4th pharyngeal pouch); secrete calcitonin; give rise to medullary carcinoma (calcitonin = tumour marker; RET mutation; MEN 2A + 2B)

Development

Descent: foramen caecum → through/past hyoid → final position C5–T1; thyroglossal duct obliterates by 8th–10th week; foramen caecum = only normal remnant

Duct passes through body of hyoid → Sistrunk’s operation excises central hyoid to prevent cyst recurrence

Congenital anomalies:

Applied Anatomy

Goitre — any thyroid enlargement regardless of cause/function

Thyroid carcinoma:

MEN associations: MEN 2A = medullary Ca + phaeochromocytoma + hyperparathyroidism; MEN 2B = medullary Ca + phaeochromocytoma + mucosal neuromas

Thyroid surgery steps (Kocher’s incision):

1. Middle thyroid vein ligated first (no artery)
2. Superior thyroid artery at upper pole capsule (protects EBSLN)
3. Inferior thyroid artery laterally (protects RLN + parathyroids)
4. RLN identified in tracheo-oesophageal groove before any posteromedial ligation
5. Parathyroids identified and preserved

Hypoparathyroidism post-thyroidectomy — most common complication of total thyroidectomy

• Tetany within 24–48 h
• Chvostek’s sign: tap facial nerve at parotid → facial twitch
• Trousseau’s sign: BP cuff above systolic 3 min → carpal spasm (main d’accoucheur)
• Rx: IV calcium gluconate (acute); oral calcium + vitamin D (long-term)

Thyrotoxicosis:

Pretibial myxoedema — raised non-pitting skin thickening over shins = pathognomonic for Graves’

Thyroid Gland — Complete Reference Note

Introduction and Special Features

The thyroid gland is the largest endocrine gland in the body, lying in the lower front and sides of the neck opposite vertebrae C5, C6, C7 and T1, clasping the upper trachea. It is H-shaped, consisting of two lateral lobes connected by an isthmus.

Hormones secreted:

• T3 (Triiodothyronine) and T4 (Thyroxine / Tetraiodothyronine) — regulate basal metabolic rate; essential for normal growth and development
• Calcitonin (Thyrocalcitonin) — lowers blood calcium by decreasing renal tubular reabsorption; secreted by parafollicular C cells; opposes parathormone

Special features:

• Only endocrine gland situated superficially — palpable and accessible for clinical examination
• Only endocrine gland dependent on the external environment (dietary iodine) for hormone synthesis
• Only endocrine gland that stores its hormones extracellularly — in follicles as colloid
• One of the most vascular organs in the body

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Parts and Dimensions

Lateral Lobes

Each lobe is conical (pyramidal) in shape:

• Apex — directed upward and slightly laterally; limited above by attachment of sternothyroid to the oblique line of thyroid cartilage
• Base — at the level of the 5th or 6th tracheal ring
• Three surfaces: anterolateral (superficial), medial, posterolateral
• Two borders: anterior (thin), posterior (thick and rounded)
• Extent: oblique line of thyroid cartilage → 5th/6th tracheal ring

Isthmus

• Connects the lower parts of both lateral lobes
• Lies in front of the 2nd, 3rd and 4th tracheal rings
• Two surfaces (anterior, posterior); two borders (superior, inferior)

Pyramidal Lobe

• Present in ~50% of individuals
• Projects upward from the isthmus, usually to the left of midline
• Represents the lower end of a persistent thyroglossal duct
• May be connected to the hyoid bone by the levator glandulae thyroideae (fibrous or fibromuscular band)

Dimensions and Weight

The gland is larger in females; increases further during menstruation and pregnancy.

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Capsules

True Capsule

• Thin fibrous layer, derived from and firmly adherent to the gland
• Sends fibrous septa (trabeculae) inward, dividing the gland into lobules, each containing 20–40 follicles
• Continuous with the fibrous stroma of the gland

False Capsule (Pretracheal Fascia Sheath)

• Derived from the pretracheal (middle) layer of deep cervical fascia
• Forms a loose sheath around the gland
• Posteriorly sends the suspensory ligament of Berry — attaches the posteromedial aspect of each lateral lobe to the cricoid cartilage and first two tracheal rings
• Anteriorly reflects over the isthmus

Space Between the Two Capsules

The loose areolar tissue between the capsules contains:

• Parathyroid glands (4 in number — 2 superior, 2 inferior)
• Terminal branches of thyroid arteries
• Tributaries of thyroid veins and anastomosing vessels

This space is the surgical plane developed during thyroidectomy to preserve parathyroids and recurrent laryngeal nerves.

Ligament of Berry — Surgical Significance

• Anchors the posteromedial thyroid lobe to the cricoid and trachea
• The recurrent laryngeal nerve passes posterior or just lateral to this ligament before entering the larynx
• Ligation near the ligament of Berry risks RLN injury → hoarseness, respiratory distress

Why Thyroid Swellings Move on Swallowing

The false capsule is derived from pretracheal fascia, which is attached superiorly to the thyroid cartilage, cricoid cartilage and hyoid bone. These structures ascend during deglutition, carrying the gland upward. All thyroid swellings therefore move upward on swallowing — the hallmark clinical sign.

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Relations

Lateral Lobe — Anterolateral (Superficial) Surface

From superficial to deep:

1. Skin
2. Superficial fascia (platysma)
3. Deep fascia (investing layer)
4. Anterior jugular veins
5. Sternohyoid muscle
6. Superior belly of omohyoid
7. Sternothyroid muscle — directly applied; its attachment to the oblique line of the thyroid cartilage limits upward lobe expansion, explaining why goitres enlarge downward (retrosternal goitre)

Lateral Lobe — Medial Surface

Lateral Lobe — Posterolateral Surface

• Common carotid artery (medially)
• Internal jugular vein (laterally)
• Vagus nerve (between artery and vein, posteriorly)
• Sympathetic trunk
• Parathyroid glands (between the capsules)

Isthmus

Recurrent Laryngeal Nerve — Critical Relation

The RLN crosses the inferior thyroid artery at the posteromedial lobe — it may pass anterior, posterior or between branches. Ligate the inferior thyroid artery laterally, never at the gland.

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Arterial Supply

The thyroid gland is one of the most vascular organs in the body.

Superior Thyroid Artery

• Descends accompanied by the EBSLN (running medial to it)
• Divides into anterior and posterior branches at the upper pole
• Ligate at the upper pole capsule — not proximally — to protect the EBSLN

Inferior Thyroid Artery

• Ascends behind the carotid sheath; loops medially at C6 (Chassaignac’s tubercle)
• Passes behind the common carotid artery and sympathetic trunk
• Supplies the parathyroid glands critically
• Ligate laterally to protect the RLN and parathyroids

Thyroidea Ima Artery

• Ascends in front of the trachea to supply the isthmus
• Must be identified before tracheostomy and during thyroidectomy — unexpected haemorrhage if divided unrecognised

Anastomoses

• Right and left superior thyroid arteries (across upper border of isthmus)
• Superior and inferior thyroid arteries on each side (posterior surface)
• Right and left inferior thyroid arteries (across isthmus inferiorly)

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Venous Drainage

Middle thyroid vein has no accompanying artery — ligated first during thyroidectomy to mobilise the lobe. Tearing it before ligation causes brisk haemorrhage from the internal jugular vein.

Inferior thyroid veins form the thyroid plexus in front of the trachea — important in tracheostomy.

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Lymphatic Drainage

First-Echelon Nodes (Levels VI and VII)

Onward Drainage

→ Deep cervical nodes (levels II–IV) → Superior mediastinal nodes (lower pole and isthmus)

Delphian node: An enlarged, hard, midline prelaryngeal swelling predicts (foretells) thyroid malignancy — named after the Oracle at Delphi.

Spread by Carcinoma Type

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Nerve Supply

The thyroid gland receives only vasomotor (sympathetic) fibres — no secretomotor innervation. Hormone secretion is regulated entirely by TSH from the anterior pituitary.

Sympathetic: Superior, middle and inferior cervical ganglia → travel along superior and inferior thyroid arteries as perivascular plexuses → vasomotor function.

Parasympathetic: Vagus (minor contribution via laryngeal branches — vasodilatory).

Nerves at Risk in Surgery

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Microscopic Structure

The thyroid is the only endocrine gland that stores its secretory product extracellularly — as colloid within follicles.

Capsule and Stroma

• Thin fibrous true capsule → septa → lobules → 20–40 follicles per lobule
• Stroma: blood vessels, lymphatics, nerves, parafollicular C cells

Follicles — Structural and Functional Units

• Spherical or ovoid closed sacs
• Wall: single layer of follicular cells (thyrocytes)
• Lumen: filled with colloid — homogeneous, eosinophilic, PAS-positive thyroglobulin

Follicular Cell Appearances

Function of follicular cells: Synthesise thyroglobulin → iodinate in colloid → reabsorb by endocytosis → cleave with proteolytic enzymes → secrete T3 and T4 into bloodstream.

Parafollicular Cells (C Cells / Light Cells)

Histological Summary

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Development

Two Sources of Origin

Timeline of Median Anlage

The thyroglossal duct passes through the body of the hyoid bone — explaining why Sistrunk’s operation excises the central portion of the hyoid to prevent cyst recurrence.

Congenital Anomalies

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Applied Anatomy

Goitre

Any enlargement of the thyroid gland regardless of cause or functional status.

Iodine-deficiency goitre (endemic): Low iodine → low T3/T4 → ↑TSH → diffuse thyroid hyperplasia. Prevention: iodised salt.

Clinical Assessment of Thyroid Swellings

• Thyroid swellings move upward on swallowing — hallmark sign (pretracheal fascia attachment to larynx/trachea)
• Thyroglossal cyst also moves on tongue protrusion (duct remnant connects to foramen caecum)

Thyroid Carcinoma

Medullary carcinoma associations:

• MEN 2A: Medullary thyroid Ca + phaeochromocytoma + hyperparathyroidism
• MEN 2B: Medullary thyroid Ca + phaeochromocytoma + mucosal neuromas

Nerve Injuries in Thyroid Surgery

Hypoparathyroidism after Thyroidectomy

Most common complication of total thyroidectomy — from inadvertent removal or devascularisation of parathyroids.

• Tetany within 24–48 hours postoperatively
• Chvostek’s sign: Tap facial nerve at parotid → facial muscle twitch
• Trousseau’s sign: BP cuff above systolic for 3 minutes → carpal spasm (main d’accoucheur)
• Treatment: IV calcium gluconate (acute); oral calcium + vitamin D (long-term)

Thyrotoxicosis (Hyperthyroidism)

Features: Weight loss despite increased appetite, heat intolerance, sweating, palpitations, tremor, anxiety, diarrhoea, goitre, exophthalmos (Graves’ only), pretibial myxoedema (Graves’ — pathognomonic; raised non-pitting skin thickening over shins).

Treatment: Antithyroid drugs (carbimazole/propylthiouracil), radioiodine (¹³¹I), surgery.

Thyroidectomy — Key Anatomical Steps

1. Kocher’s incision — transverse skin crease, 2 cm above sternal notch
2. Sternothyroid divided or retracted (limits upward mobilisation)
3. Middle thyroid vein ligated first (no accompanying artery) → allows lobe mobilisation
4. Superior thyroid artery ligated at upper pole capsule (protects EBSLN)
5. Inferior thyroid artery ligated laterally (protects RLN and parathyroids)
6. RLN identified in tracheo-oesophageal groove before any posteromedial ligation
7. Parathyroid glands identified and preserved with their vascular supply

Thyroid Isthmus and Tracheostomy

• Isthmus covers 2nd–4th tracheal rings
• Low (collar) tracheostomy: isthmus must be divided and ligated
• Thyroidea ima artery (if present) lies in front of the trachea — identify before dividing

Investigations

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