THYROID GLAND PRODUCTS

T3 (full name, triiodothyronine)

• T3 is more biologically active.

T4 (full name, thyroxine, aka, tetraiodonthyronine).

• The thyroid produces T4 in greater quantities; so, target tissues to need to use 5′ iodinase convert T4 to T3.

THYROID HORMONE SYNTHESIS

• Synthesis occurs both intra- and extracellularly.

Step 1:

• Thyroglobulin is synthesized in the follicular epithelial cell and transported to the lumen.
  • Thyroglobulin is a tyrosine-rich protein.

Step 2:

• The “i-trap,” which is a sodium-iodine co-transporter, pulls iodiDe into the cell from the capillaries.
  • Iodide is a trace element that does not occur naturally in the body, so it must be consumed in the diet.

Step 3:

• Oxidization of iodiDe to iodiNe by the enzyme thyroid peroxidase.

Step 4:

• Organification, also driven by thyroid peroxidase, to combine iodine with the tyrosine of luminal thyroglobulin;
• As a result of organification, two thyroid hormone precursors form and attach to thyrogobulin:
  • MIT (full name, monoiodotyrosine)
  • DIT (full name, diiodotyrosine)

Step 5:

• Thyroid peroxidase drives coupling reactions:
  • Two DIT molecules combine to form T4.
  • One DIT molecule combines with one molecule of MIT to form T3.
• Ultimately, some MIT and DIT will be “left over,” and remain bound to thyroglobulin with T3 and T4 in the colloid.
• Recall that, as we learned earlier, T4 is produced in larger quantities.

Step 6:

• Thyroglobulin, along with thyroid hormone and its precursors, are endocytosed to the follicular cell.

Step 7:

• Upon glandular stimulation, MIT and DIT are released from thyroglobulin; they remain within the cell to be recycled in the synthesis of new thyroglobulin.
• Though omitted for simplicity, MIT and DIT are deiodinated
  • IoDide returns to the pool of iodide within the cell.
  • Tyrosine molecules are recycled in the synthesis of new thyroglobulin molecules.
• T3 and T4 are delivered to the systemic circulation to reach their target tissues.
• Most T3 and T4 travels in the blood bound to thyroxine-binding globulin (a carrier protein); only free T3 and T4 are physiologically active.