Synthesis of Thyroid Hormone Lecture Notes

Introduction

• Thyroid Gland Location: Anterior neck, inferior to larynx, butterfly-shaped.
• Structure: Contains thyroid follicles, the structural and functional units.

Hypothalamic-Pituitary-Thyroid (HPT) Axis

• Hypothalamus: Begins hormone production.
  • Produces Thyrotropin Releasing Hormone (TRH) from paraventricular nucleus.
• Anterior Pituitary: Stimulated by TRH.
  • Thyrotropes release Thyroid Stimulating Hormone (TSH).
• Thyroid Gland: TSH stimulates thyroid follicles.

Mechanism of Thyroid Hormone Synthesis

Step 1: TRH Release

• From hypothalamus paraventricular nucleus.

Step 2: TSH Release

• From anterior pituitary thyrotropes.

Step 3: Thyroglobulin Synthesis

• Stimulated by TSH in thyroid follicles.

Step 4: Iodide Trapping

• Iodide (I-) from blood into follicular cells by sodium co-transport (secondary active transport).

Step 5: Oxidation of Iodide

• Enzyme: Thyroid peroxidase (TPO) converts iodide to iodine (oxidation).

Step 6: Iodination

• Iodine binds to tyrosine residues on thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT).

Step 7: Coupling

• DIT + DIT = T4 (Thyroxine)
• MIT + DIT = T3 (Triiodothyronine)

Step 8: Endocytosis

• Thyroglobulin with T3 and T4 is internalized into follicular cells.

Step 9: Cleavage by Lysosomal Enzymes

• T3 and T4 are cleaved from thyroglobulin.

Step 10: Exocytosis

• T3 and T4 are released into the bloodstream.
• Transported by Thyroxine Binding Globulin (TBG).

Summary

• The HPT axis regulates the synthesis process.
• Key enzymes include thyroid peroxidase and transporter proteins like pendrin.
• Thyroid hormone (T3 and T4) impacts target organs, further discussed in the next lecture.