Thyroid Hormone Synthesis Pathway

Overview

• The video explains the thyroid hormone synthesis pathway step-by-step.
• Knowing these steps is crucial for diagnosing and treating thyroid conditions.
• The pathway involves the hypothalamus, anterior pituitary gland, and thyroid gland.
• These form the hypothalamic-pituitary-thyroid axis (HPT axis) which regulates thyroid hormone levels.

Structures Involved

1. Hypothalamus

   • Located below the thalamus in the brain.
   • Secretes thyrotropin-releasing hormone (TRH).

2. Anterior Pituitary Gland

   • Located below the hypothalamus.
   • TRH stimulates it to release thyroid-stimulating hormone (TSH).

3. Thyroid Gland

   • Located at the base of the neck, butterfly-shaped.
   • TSH stimulates it to produce thyroid hormones.

Hormone Pathway

1. Hypothalamus to Anterior Pituitary

   • TRH is secreted by the paraventricular nucleus in the hypothalamus.
   • TRH travels to the anterior pituitary gland via the hypophyseal portal system.
   • Stimulates thyrotropes in the anterior pituitary to release TSH.

2. Anterior Pituitary to Thyroid Gland

   • TSH enters the bloodstream and stimulates the thyroid gland.
   • The gland synthesizes thyroid hormones (T3 and T4).

Thyroid Hormone Synthesis

• Thyroid Hormones: Triiodothyronine (T3) and Thyroxine (T4).
• Thyroid Gland Structure: Comprised of follicles with thyrocytes lining and colloid in the center.

Synthesis Steps

1. TSH Binding:

   • TSH binds to receptors on thyroid follicular cells.
   • Stimulates production of thyroglobulin and uptake of iodide from blood.

2. Iodide Handling:

   • Iodide is absorbed from diet and converted into iodine by the enzyme thyroid peroxidase (TPO).

3. Iodination and Coupling:

   • Iodine is added to tyrosine residues in thyroglobulin to form MIT (Monoiodotyrosine) and DIT (Diiodotyrosine).
   • TPO facilitates the coupling of MIT and DIT to form T3 and T4.

4. Release into Bloodstream:

   • T3 and T4 are cut from thyroglobulin and released into bloodstream.
   • Travel bound to thyroxine-binding globulin (TBG).

Key Enzymes

• Thyroid Peroxidase (TPO):
  • Converts iodide to iodine (oxidation).
  • Facilitates iodination of thyroglobulin.
  • Couples MIT and DIT to form T3 and T4.

Important Proteins

• Thyroglobulin: Precursor to thyroid hormones.
• Thyroxine-Binding Globulin (TBG): Transports T3 and T4 in the blood.

Conclusion

• The pathway is crucial for understanding thyroid-related conditions.
• Further details can be found in the lecture notes and study guide linked in the video description.
• Future videos will cover the role of these hormones and proteins in diagnosing and treating thyroid conditions.