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Thyroid gland anatomy

Overview

  • Two symmetrical lobes
  • United by an Isthmus anterior to 2nd, 3rd and 4th Tracheal Rings
  • Lobes:
    • Lie on either side of Trachea and Larynx
    • Extends from Oblique Line of Thyroid Cartilage to 6th Tracheal Ring
  • Weight: Approx 25g
  • Enclosed by own capsule AND Pretracheal Fascia

Lateral Lobe

  • Narrow upper pole and broader lower pole
  • Triangular in cross section
  • Lateral, medial and posterior surfaces
  • Lateral surface:
    • Deep to Sternothyroid and Sternohyoid
  • Medial surface:
    • Lies against lateral side of Larynx and upper Trachea
    • Lower Pharynx and Upper Oesophagus immediately behind
    • Related to:
  • Posterior surface:
    • Overlaps medial part of Carotid Sheath (i.e. the part containing the Common Carotid Artery)
    • If enlarged, the lobe may extend across the more laterally placed Internal Jugular Vein
    • In contact with Parathyroid Glands – between it and the Fascial Sheath

Tubercles of Zuckerkandl

  • Lateral or posterior projection from the lateral lobe of the Thyroid
  • Originates from the ventral portion of the Ultimobranchial Body
  • In > 90% of cases, Recurrent Laryngeal Nerve runs posterior to the Tubercle, either in:
    • Tracheo-oesophageal Groove (60%)
    • Posterior (24%)
    • Lateral (5%) to Trachea
  • Only a small percentage (¬5%) of cases does the Recurrent Laryngeal Nerve run anterior to the Thyroid – usually re-op surgeries
  • Relationship to Inferior Thyroid Artery variable:
    • 85% - posterior and intertwined between branches
    • 15% - anterior (more common on the right)

Recurrent Laryngeal Nerve

External Laryngeal Nerve

Cernea Classification

Categorizes relationship between External Laryngeal Nerve and Superior Thyroid Artery [WB] Type 1: - Nerve crosses the Superior Thyroid Vessels > 1cm above the upper pole of Thyroid Type 2a: - Crosses the Superior Thyroid vessels < 1cm above the upper pole of Thyroid Type 2b: - Crosses at or below level of upper pole of Thyroid

Isthmus

  • Joins anterior parts of the lobes towards their lower poles
  • Posterior surface firmly adherent to 2nd, 3rd and 4th Tracheal Rings
    • Pretracheal Fascia here fixed between them
    • This fixation and the investment of the whole gland by Pretracheal Fascia is responsible for the gland to move up and down with the Larynx during swallowing
  • An anastomosis between the two Superior Thyroid Artery runs across the upper border of the Isthmus
  • Tributaries of the Inferior Thyroid Veins emerge from its lower border

Pyramidal Lobe

  • Small portion of gland substance projecting upwards from the Isthmus
    • Generally to the left of midline
  • Represents a development of glandular tissue from the caudal end of the Thyroglossal Duct
  • May be attached to the inferior border of the Hyoid Bone by fibrous tissue
    • Levator Glandulae Thyroidae – muscle fibres present within this fibrous tissue
      • Nerve supply: External Laryngeal Nerve

Accessory Thyroid Glands

  • Not uncommonly found near:
    • Hyoid
    • In the Tongue
    • In Superior Mediastinum
    • Anywhere along the path of descent of the Thyroglossal Duct

Strap Muscles

  • Consists of:

    • Sternohyoid
    • Omohyoid
    • Sternothyroid
    • Thyrohyoid

  • Nerve Supply:

    • Sternohyoid – Anterior rami of spinal nerves (C1-C3) via Ansa Cervicalis
    • Omohyoid – Superior root of Ansa Cervicalis (Superior belly) and Ansa Cervicalis (Inferior Belly)
    • Sternothyroid – Anterior rami of C1-C3 via the Ansa Cervicalis
    • Thyrohyoid – Branch of Hypoglossal Nerve (but all fibres are hitch-hiking from C1)

Blood Supply

Superior Thyroid Artery

  • Origin
  • Course
    • Pierces Pretracheal Fascia
    • Runs almost vertically downwards, with the vein, to the upper pole of Thyroid
    • Runs with the External Laryngeal Nerve
    • 4 branches prior to reaeching the upper pole
      • A single vessel reaches summit of upper pole
    • Artery divides on the gland into anterior and posterior glandular branches
      • Anterior branch – runs down to Isthmus
      • Posterior branch – runs down the back of the lobe and anastomoses with an ascending branch of the Inferior Thyroid Artery from the lower pole
    • In thyroidectomy – artery ligated close to upper pole or its anterior and posterior branches are ligated instead – avoid damage to External Laryngeal Nerve
  • Branches
    • Before reaching Thyroid, it gives off
      • Infrahyoid branch
      • SCM brnach
      • Superior Laryngeal branch
      • Cricothyroid branch
        • Cricothyroid Artery crosses the upper part of the Cricothyroid membrane to anastomose with the contralateral artery

Inferior Thyroid Artery

  • From Thyrocervical Trunk
  • Arches upwards and medially behind the Carotid Sheath
  • Loops downwards to lower pole
  • Divides outside Pretracheal Fascia into branches that pierce the fascia separately to reach the lower part of the gland
  • Recurrent Laryngeal Nerve has variable relationship with artery
    • BUT always lies behind the Pretracheal Fascia
    • 85% - posterior and intertwined between branches
    • 15% - anterior (more common on the right)
    • To safeguard the nerve, ligate the Inferior Thyroid Artery well lateral to the gland or carefully ligate its small branches on the surface of the gland
  • Branches
    • Ascending Cervical Artery (or this can come off Thyrocervical Trunk)
    • Small Pharyngeal branches
    • Oesophageal branches
    • Tracheal branches
    • Inferior Laryngeal artery
      • Inferior Laryngeal Artery ascends with the Recurrent Laryngeal Nerve

Thyroidea Ima Artery

  • Arises from Brachiocephalic Trunk, Arch of the Aorta or the Right Common Carotid Artery
  • Enters the lower part of the Isthmus in 3% of individuals

Venous Drainage

  • Venous plexus on gland’s surface

Thyroid veins

  • Superior thyroid vein
  • Middle thyroid vein
    • Crosses anterior to Common Carotid Artery
    • Drains into the Internal Jugular Vein
    • Early ligature and division of this vein during thyroid surgery facilitates mobilisation of the gland
  • Inferior Thyroid Vein
    • Drain downwards mainly into Left Brachiocephalic Vein – one may enter the Right Brachiocephalic Vein

Lymph Drainage

  • Mainly to Deep Cervical nodes
  • Few pass into Pre-Laryngeal, Pre and Para-Tracheal Nodes and directly into the Thoracic Duct

Nerve Supply

  • Sympathetic (vasoconstrictor) nerves from Superior, Middle and Inferior Cervical Ganglia – accompany Thyroid Arteries

Structure

  • Thyroid made up of rounded follicles containing varying amounts of colloid produced by a single layer of Epithelial (Follicular) cells that form the walls of the follicles
  • Only endocrine gland to store its secretions outside the cells
  • Colloid is iodinated when in the follicle
  • Reabsorbed by the cells before being discharged into blood capillaries
  • Main hormonal products: Thyroxine (T4) and Triiodothyronine (T3)
  • Parafollicular Cells or C-Cells - make up < 2% of the epithelial cells
    • Secrete Calcitonin
    • Scattered on outer aspects of follicles
    • Do not reach the lumina of the follicles

Embryology of thyroid

  • Develops as a proliferation of cells from the caudal end of the Thyroglossal Duct
  • Thyroglossal Duct
    • Originates from the Endoderm of the Floor of the Pharynx at the Foramen Caecum in the region of the developing tongue
    • Then passes caudally in front of the Hyoid Bone, behind which it forms a recurrent loop
    • Thyroid gland buds from the duct’s distal end
      • Distal end, itself, may give rise to the Pyramidal Lobe
    • Other remnants of the duct may persist as Accessory Thyroid Glands or Thyroglossal cyst
  • Removal of Retrohyoid Duct Remnants usually requires excision of a segment from the middle of the Hyoid Bone]
  • Failure of descent of the Thyroglossal Duct may result in the development of a Lingual Thyroid
  • Parafollicular Calcitonin-producing cells develop from the Ultimobranchial Body (5th Pharyngeal Pouch) under influence of Neural Crest Cells

Surgical Approach

  • Transverse incision in a low skin crease at the front of the neck
    • 2 finger-breadths above the clavicle and between the sternocleidomastoids
  • Layers divided: Skin ⇒ Subcutaneous Fat ⇒ Platysma ⇒ Develop superior platysmal flap by dissecting in subplatysmal plane ⇒ Raise upper flap as far as Thyroid Cartilage + Raise lower flap as far as Sternal Notch ⇒ Incise Investing Layer of Deep Cervical Fascia vertically ⇒ Incise Midline Raphe between strap muscles ⇒ Strap muscles retracted laterally ⇒ Incise Pretracheal Fascia
  • Sternohyoid and Sternothyroid muscles retracted
    • Or divided at a high level to safeguard their nerve supply from the Ansa Cervicalis and prevent adherence of their subsequent suture line to the closure of the skin incision
  • Pretracheal Fascia divided to expose the gland proper

Structures Spared During Initial Exposure

  • Strap muscles
  • Anterior Jugular Veins

Structures Divided During Initial Exposure

  • Platysma
  • Midline Raphe

Graph View

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