LG 6

THYROID GLAND STUDY NOTES

Definition of Thyroid Gland

  • Description: The thyroid gland is a bi-lobed endocrine gland located at the midline of the neck.
  • Function: It secretes hormones that regulate metabolism, growth, and development.

Hormonal Regulation

  • Thyroid Hormones:
    • TRH: Thyroid Releasing Hormone (produced by the hypothalamus)
    • TSH: Thyroid Stimulating Hormone (produced by the pituitary gland)
    • T3: Triiodothyronine hormone
    • T4: Thyroxine hormone
    • Calcitonin: Hormone involved in calcium homeostasis
  • Feedback Mechanism: The secretion of T3 and T4 is regulated by negative feedback inhibition involving TRH and TSH.

Functions of the Thyroid Gland

  1. Growth and Development: Plays a crucial role in physical development.
  2. Heart Rate Regulation: Stimulates an increase in heart rate.
  3. Cardiac Function: Enhances the strength of heart contractions.
  4. Nutrient Synthesis: Stimulates the synthesis of proteins and carbohydrates.
  5. Lipid Metabolism: Degrades cholesterol and triglycerides.
  6. Beta-adrenergic Receptors: Enhances these receptors to respond to catecholamines (e.g., adrenaline).
  7. Increased Vitamin Requirements: Elevates the need for vitamins in metabolic processes.

Location and Description of the Thyroid Gland

  • Lobes: Composed of right and left lobes connected by an isthmus.
  • Lobe Characteristics:
    • Shape: Pear-shaped
    • Apex: Directed upward to the oblique line of the thyroid cartilage
    • Base: Lies below at the level of the 4th or 5th tracheal ring.
Isthmus of the Thyroid Gland
  • Definition: Connects the right and left lobes of the thyroid gland.
  • Location: Extends across the midline in front of the 2nd, 3rd, and 4th tracheal rings.
Pyramidal Lobe
  • Presence: Often present, projecting upward from the isthmus, usually to the left of midline.
  • Connection: Can connect to the hyoid bone via either a fibrous or muscular band (referred to as the levator glandulae thyroideae).
Structural Sheath
  • Sheath Composition: The thyroid gland is surrounded by a connective tissue sheath.
  • Attachment: The sheath attaches the gland to the larynx and trachea.
  • Fascia Type: Pretracheal layer of deep fascia envelops the gland.

Capsules of Thyroid Gland

  1. True Capsule: Made of the gland's own connective tissue.
  2. False Capsule:
    • Origin: Derived from the pretracheal layer of deep cervical fascia.
    • Characteristics: Thin along the posterior border but thick on the inner surface.
    • Function: Forms a suspensory ligament (of Berry) connecting the lobe to the cricoid cartilage.

Blood Supply to the Thyroid Gland

  • Arteries:
    1. Superior Thyroid Artery:
    • Origin: Branch of the external carotid artery.
    • Path: Descends to the upper pole of each lobe, accompanied by the external laryngeal nerve.
    1. Inferior Thyroid Artery:
    • Origin: Branch of the thyrocervical trunk.
    • Path: Ascends behind the common carotid artery to the cricoid cartilage, turns medially and downward to reach the posterior border of the gland.
    1. Thyroidea Ima Artery (if present):
    • Origin: Can arise from either the brachiocephalic artery or the arch of the aorta.
    • Path: Ascends in front of the trachea to the isthmus.

Venous Drainage from the Thyroid Gland

  1. Superior Thyroid Vein: Drains into the internal jugular vein (IJV).
  2. Middle Thyroid Vein: Drains into IJV.
  3. Inferior Thyroid Veins: Anastomose with each other as they descend in front of the trachea, draining into the left brachiocephalic vein in the thorax.

Lymphatic Drainage

  • Nodes Involved:
    • Prelaryngeal nodes
    • Pretracheal nodes
    • Paratracheal nodes
  • Subsequent Drainage:
    • Drainage to superior deep cervical nodes or inferior deep cervical nodes.

Nerve Supply to the Thyroid Gland

  1. Sympathetic Nerves:
    • Origin: From superior, middle, and inferior cervical ganglia for vasomotor supply.
  2. Parasympathetic Nerves:
    • Origin: From the vagus and recurrent laryngeal nerves.

Relations of the Lobes of the Thyroid Gland

  • Anterolateral Relations:
    • Skin
    • Platysma
    • Anterior jugular veins
    • Investing layer of deep cervical fascia
    • Sternothyroid
    • Omohyoid (superior belly)
    • Sternohyoid
    • Sternocleidomastoid (anterior border)
Posterolateral Relations
  • Associated Structures:
    • Carotid sheath containing:
    • Common carotid artery
    • Internal jugular vein
    • Vagus nerve
    • Prevertebral layer of deep cervical fascia
    • Esophagus and sympathetic trunk.
Medial Relations
  1. Larynx / Trachea
  2. Pharynx / Esophagus
  3. Cricothyroid muscle and its nerve supply (external laryngeal nerve)
  4. Recurrent laryngeal nerve, in the groove between the esophagus and trachea
  5. Superior and inferior parathyroid glands
  6. Anastomosis between the superior and inferior thyroid arteries.
Relations of the Isthmus
  • Anterior Relations:
    • Sternothyroids
    • Sternohyoids
    • Anterior jugular veins
    • Fascia and skin
  • Posterior Relations:
    • 2nd, 3rd, and 4th rings of the trachea
    • Termal branches of the superior thyroid arteries anastomose along its upper border.