Overview
This lecture covered the external anatomy, parts, and important surgical relations of the thyroid gland, with emphasis on surface anatomy and clinical relevance.
Location and Surface Anatomy
- The thyroid gland is shield-shaped and lies low in the front of the neck, below and lateral to the thyroid cartilage.
- It is not at the laryngeal prominence (Adam’s apple) but below it; vertebral level C5 to T1.
- Weighs 20–25 grams, depending on body weight and iodine intake.
- Highly vascular, brown, firm, and secretes thyroxine (regulates metabolism) and calcitonin (lowers blood calcium).
Parts and Structure
- The gland is H-shaped: two lobes connected by a central isthmus.
- Isthmus lies in front of the 2nd-4th tracheal rings; lobes lie on either side of larynx and trachea.
- Each lobe extends from the oblique line of thyroid cartilage to the sixth tracheal ring.
- Pyramidal lobe (present in ~30%): an upward extension from the isthmus, sometimes connected to the hyoid bone by the levator glandulae thyroideae (fibrous band).
- The pyramidal lobe is a remnant of the thyroglossal duct.
Capsule and Fascia
- Covered by two capsules: true capsule (dense fibrous, adherent) and false capsule (derived from pretracheal fascia, easily separated).
- Venous plexus lies inside true capsule (contrast: in prostate, it's outside true capsule).
- Ligament of Berry: dense posterior-medial condensation attaching gland to cricoid cartilage and first tracheal ring; key fixation and surgical landmark.
Relations and Surfaces
- Apex of lobes reaches oblique line of thyroid cartilage, limited by sternothyroid muscle.
- Base of gland at 5th/6th tracheal ring, related to inferior thyroid artery and recurrent laryngeal nerve.
- Lateral (superficial) surface covered by sternothyroid, sternohyoid, and superior belly of omohyoid muscles.
- Medial surface contacts trachea, esophagus, inferior constrictor, cricothyroid muscles, cricoid and thyroid cartilage, and is near external and recurrent laryngeal nerves.
- Posterolateral surface related to parathyroid gland and carotid sheath.
- Anterior border related to superior thyroid artery; posterior border to arterial anastomoses between superior and inferior thyroid arteries.
Isthmus
- Connects the lobes, has anterior and posterior surfaces, superior and inferior borders.
- Anteriorly related to sternohyoid, sternothyroid muscles, and anterior jugular vein.
- Posterior surface related to 2nd, 3rd, 4th tracheal rings.
- Superior border: arterial anastomosis (superior thyroid arteries); inferior border: exit for inferior thyroid vein, possible entry of thyroid ima artery.
Clinical and Surgical Relevance
- Thyroid moves with swallowing due to connection with laryngeal cartilages and hyoid bone via pretracheal fascia.
- Ligament of Berry is crucial in surgery due to proximity to recurrent laryngeal nerve and inferior thyroid artery branch.
Key Terms & Definitions
- Isthmus — central part connecting right and left thyroid lobes.
- Pyramidal lobe — an upward extension of thyroid tissue, remnant of the thyroglossal duct.
- True capsule — dense, adherent fibrous covering of the gland.
- False capsule — loose covering from pretracheal fascia, easily separated.
- Ligament of Berry — dense band fixing thyroid to cricoid cartilage, key in gland fixation.
- Thyroxine — hormone increasing cell metabolism.
- Calcitonin — hormone lowering blood calcium.
Action Items / Next Steps
- Review diagrams of thyroid anatomy and relations.
- Practice palpation of the thyroid gland below the Adam’s apple.
- Know the surgical relevance of ligament of Berry and surface anatomy for exams.