Endocrine: Thyroid Anatomy & Physiology

Thyroid Anatomy

Embryology

  • Thyroid Tissue: Endoderm from Foramen Cecum
    • Descends into Neck
  • C Cells: Neural Crest from Fourth Pharyngeal Pouch

Structure

  • Right & Left Lobes
  • Isthmus
    • Connects Right & Left Lobes
  • Pyramidal Lobe (Lalouette’s Pyramid)
    • Remnant of Thyroglossal Duct
    • Extends Superiorly from the Isthmus
  • Zuckerkandl Tubercles
    • Posterolateral Horns
    • Overlies the Recurrent Laryngeal Nerves

Suspensory Ligaments

  • Anterior Suspensory Ligament
    • Suspends Thyroid from Cricoid & Thyroid Cartilage
    • Contains Delphian Nodes
  • Posterior Suspensory Ligament (Ligament of Berry)
    • Suspends Thyroid from the Trachea
    • Close to the Recurrent Laryngeal Nerves

Vascular Supply

  • Arteries:
    • Superior Thyroid Artery
      • From: External Carotid Artery (First Brach)
      • Near the Superior Laryngeal Nerve Mn
    • Inferior Thyroid Artery
      • From: Thyrocervical Trunk (Off Subclavian Artery)
      • Near the Recurrent Laryngeal Nerve
        • ITA Usually Anterior to RLN on the Right
        • ITA Usually Posterior to RLN on the Left
      • Also Supplies Both Superior & Inferior Parathyroids
    • Thyroidea Ima Artery
      • From: Innominate (Most Common), Aorta or Carotid Arteries
      • Supplies the Isthmus
      • Present in 3-5% of the Population
  • Veins:
    • Superior Thyroid Vein – Drains to the Internal Jugular Vein
    • Middle Thyroid Vein – Drains to the Internal Jugular Vein (Same as Superior)
    • Inferior Thyroid Vein – Drains to the Innominate Vein

Associated Nerves

Thyroid Structure 1

Thyroid Vasculature 2

Laryngeal Nerves Associated with the Thyroid Gland (Posterior View) 3

Thyroid Hormones

Thyroid Cells

  • Thyroid Follicles
    • Spherical Groupings of Thyrocytes/Follicular Cells – Produce T3/T4 Hormone
    • Surrounds a Colloid Core of Thyroglobulin Proteins
  • Parafollicular C Cells
    • Scattered Between Spherical Follicles
    • Produce Calcitonin

Thyroid Hormone Production

  • Precursors:
    • Thyroglobulin (Tg)
      • Precursor to T3/T4
      • Produced in Follicular Cells
      • Stored in Colloid
    • Iodide
      • Iodine Travels in the Blood as Iodide
      • Actively Transported into Follicular Cells by Na/I Symporter
      • Passively Transported into Colloid by Pendrin
  • Thyroid Production:
    • Thyroperoxidase (TPO)
      • Stored in Colloid
      • Oxidizes Iodide to Active Iodine
    • Oxidized Iodine then Iodinates Tyrosine Units of Thyroglobulin
    • Iodinated Thyroglobulin Units form Precursors for T3/T4 (Monoiodotyrosine/MIT & Diiodotyrosine/DIT)
    • Thyroid Stimulating Hormone (TSH)
      • Stimulates Iodinated Thyroglobulin Endocytosis/Digestion to T3/T4
    • T3 (10-20%) & T4 (80-90%) are then Secreted
  • Peripheral Conversion:
    • Deiodinase
      • Converts T4 to T3 in Periphery
    • Thyroid Binding Globulin
      • Transports Thyroid Hormones
      • Binds the Majority of Peripheral T3/T4

Thyroid Hormones

  • Triiodothyronine (T3)
    • Most Active than T4
    • Majority (87%) is Produced in the Periphery
    • Effects:
      • Increased Heart Rate & Cardiac Output
      • Increased Respiratory Rate
      • Increased Basal Metabolic Rate
      • Potentiates Catecholamine Effects
  • Thyroxine (T4)
    • 15x More Prevalent than T3
    • Less Active than T3
    • Similar Effects as T3
  • Calcitonin
    • Overall Effects: Decreases Calcium & Phosphate Mn
    • Bone:
      • Decreases Osteoclast Calcium Secretion
    • Renal:
      • Decreases Calcium Reabsorption
      • Decreases Phosphate Reabsorption
    • Most Common Symptom of Oversecretion: Diarrhea

Thyroid Hormone Synthesis 4

Mnemonics

Relationship of Thyroid Arteries to Laryngeal Nerve

  • Superior-Superior
  • Inferior-Recurrent

Calcitonin Effects on Calcium

  • Calci-tone-in: Tones Down Calcium

References

  1. Wikimedia Commons. (License: Public Domain)
  2. Arrangoiz, R. , Cordera, F. , Caba, D. , Muñoz, M. , Moreno, E. and de León, E. (2018) Comprehensive Review of Thyroid Embryology, Anatomy, Histology, and Physiology for Surgeons.International Journal of Otolaryngology and Head & Neck Surgery, 7, 160-188. (License: CC BY-SA-4.0)
  3. Wikimedia Commons. (License: Public Domain)
  4. Haggstrom M. Wikimedia Commons. (License: Public Domain)