Thyroid Gland and Hormone Production

Location and Structure of Thyroid Gland

• Location: Inferior and anterior to the larynx (throat area).
• Shape: Bow tie-like, with two lobes connected by an isthmus.
• Microscopic Structure: Composed of segments called follicles.
  • Follicles: The site of thyroid hormone production.

Hormones Produced

• Triiodothyronine (T3)
• Tetraiodothyronine (T4) / Thyroxine
• Proportion: 90% T4, 10% T3
• Potency: T3 is about 4 times more potent than T4.

Thyroid Follicles

• Filled with colloid (protein-rich material)
• Follicular Cells:
  • Secrete proteins into colloid.
  • Associated with capillaries for iodide delivery and hormone release.
  • Have polarized membranes:
    • Apical membrane faces the colloid.
    • Basolateral membrane faces the capillaries.

Hormone Synthesis Process

Iodide Trapping

• Source: Iodide from the diet.
• Transport: Sodium iodide symporter at the basolateral membrane (secondary active transport).

Iodide Oxidation

• Enzyme: Thyroid peroxidase
• Process: Oxidizes iodide anion to iodine.

Thyroglobulin Synthesis

• Synthesized in follicular cells.
• Travels from ER → Golgi body → secretory vesicles.
• Released into colloid where it becomes iodinated at tyrosine residues.

Hormone Formation

• T1 (one iodine) + T2 (two iodines) -> T3 (Triiodothyronine)
• Two T2 molecules -> T4 (Thyroxine)

Storage and Release

• Stored in colloid attached to thyroglobulin.
• Released via endocytosis and fusion with lysosome to form endolysosome.
• Enzymes in lysosome: Break down thyroglobulin, releasing T3 and T4.
• Entry into bloodstream: Diffuse across basolateral membrane.

Transport in Bloodstream

• Require transporter proteins (e.g., thyroxine-binding globulins).
• Over 99% bound to transport proteins in blood.

Peripheral Conversion and Cellular Action

• Conversion: T4 converted to T3 in peripheral tissues by monodeiodinase.
• Action: T3 binds to a thyroid hormone receptor in the nucleus and initiates transcription and translation, e.g., for sodium-potassium pump production.