Thyroid Gland - structure, function

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Thyroid Gland

1. Anatomy and Location

• Largest Endocrine Gland: Butterfly-shaped, located inferior to the larynx and anterior to the trachea.
• Structure:
  • Two lobes (left and right) connected by the isthmus.
  • Highly vascularized, giving it a reddish hue.
• Blood Supply:
  • Arteries: Superior and inferior thyroid arteries.
  • Veins: Superior, middle, and inferior thyroid veins.

2. Structure of the Thyroid Gland

• Fibrous Capsule: Encloses the gland.
• Thyroid Follicles:
  • Spherical structures composed of:
    • Follicular cells: Form the wall of the follicle.
    • Follicle lumen: Central cavity containing a viscous colloid with thyroglobulin.
  • Parafollicular cells (C cells): Located outside the follicles.
    • Function: Secrete calcitonin, which lowers calcium levels and opposes parathyroid hormone.

3. Thyroid Hormones (TH)

• Two major hormones:
  • T3 (Triiodothyronine).
  • T4 (Tetraiodothyronine or Thyroxine).
• Regulation:
  • Synthesis and secretion maintained by thyroid-stimulating hormone (TSH) from the anterior pituitary.

4. Thyroid Hormone Production

Phase 1: Formation of Iodinated Thyroglobulin

• Follicular cells secrete thyroglobulin (rich in tyrosine) into the follicle lumen.
• Iodide (I⁻) from the blood is converted into iodine (I) in the lumen.
• Iodine binds to tyrosine in thyroglobulin, forming iodinated thyroglobulin, stored in the lumen for months.

Phase 2: Synthesis of T3 and T4

• Iodinated thyroglobulin is absorbed back into follicular cells by endocytosis.
• Lysosomal enzymes hydrolyze the thyroglobulin to release T3 and T4, which enter the bloodstream.
• Primary Effect: Regulates cellular metabolism.
• In Children: Stimulates growth and development through the release of growth hormone.

5. Disorders of Thyroid Hormone Production

• Hyperthyroidism (Excess TH):
  • Symptoms:
    • High metabolic rate.
    • Weight loss.
    • Hyperactivity.
    • Heat intolerance.
    • Goiter (enlarged thyroid).
• Hypothyroidism (Insufficient TH):
  • Symptoms:
    • Low metabolic rate.
    • Weight gain.
    • Sluggishness.
    • Sensitivity to cold.