ESR [018] Thyroid Axis lecture 2024 2025

Introduction

  • Title: Towards Unbounded Thinking

  • Institution: NGU School of Medicine

The Normal Thyroid Axis

  • Focus on the biosynthesis of thyroid hormones and their mechanisms of action.

  • Overview of the session's objective, aimed at informing NGU students about thyroid gland morphology and function.

Session Objectives

  • By the end of the session, students should be able to:

    • Describe with diagrams the gross morphology of the thyroid gland.

    • Sketch the structure of thyroid follicles and relate them to T3 and T4 biosynthesis.

    • List circulating binding proteins involved in thyroid hormone transport.

    • Explain why T4 is regarded as a prohormone.

    • Discuss T3 receptors and detail the physiological actions of T3.

Anatomy and Morphology of the Thyroid

Gross Structure

  • The thyroid is butterfly-shaped with:

    • Two lobes connected by an isthmus.

    • Average weight: 15-40g.

    • Location: Below larynx, in front of trachea.

    • Enlargement condition: Goitre.

Parathyroid Glands

  • Four parathyroid glands are located on the posterior surface of the thyroid.

  • Function: Sensing calcium levels and secreting Parathyroid Hormone (PTH).

Follicular Structure

  • Thyroid gland contains multiple acini (follicles):

    • Each follicle is spherical, lined by epithelial cells, and filled with colloid (predominantly thyroglobulin).

    • Colloid composition is crucial for hormone synthesis.

Thyroid Hormone Synthesis

Types of Hormones

  • Thyroid hormones produced:

    • T4 (Thyroxine) and T3 (Triiodothyronine):

      • Primary secretion: T4, with T3 having higher biological activity.

      • T3 is formed from deiodination of T4 in peripheral tissues.

    • Calcitonin from C-cells, which antagonizes PTH and decreases blood calcium levels.

Process of Hormone Production

  1. Iodine Transport

    • Active transport of iodide (I-) from blood into follicular lumen.

    • High concentration of iodide relative to other body tissues.

  2. Thyroglobulin Synthesis

    • Contains tyrosine amino acids precursors for thyroid hormones.

    • Synthesized in follicular epithelial cells and secreted into the follicular lumen.

  3. Role of Thyroid Peroxidase (TPO)

    • Key enzyme generating iodine (I2) from iodide.

    • Links iodine with tyrosine residues in thyroglobulin to form MIT (monoiodotyrosine) and DIT (diiodotyrosine) through organification.

    • TPO aids in the coupling process, primarily producing T4, with a lesser amount of T3.

  4. Endocytosis and Release

    • Endocytosis of thyroglobulin occurs when the thyroid is stimulated for hormone release.

    • Hormones are released upon thyroglobulin destruction by lysosomes within follicular cells.

Regulation and Action of Thyroid Hormones

Hormonal Regulation

  • TSH (Thyroid-stimulating hormone) stimulates thyroid function and hormone production:

    • Activates cell metabolism and iodide uptake.

    • Determines the rate of T3 and T4 synthesis.

Physiological Effects

  • T3 is more bioactive than T4, affecting various physiological processes:

    • Increases basal metabolism and promotes growth and development.

    • Impacts cardiovascular and central nervous system functions.

    • Modulates the actions of other hormones, including adrenaline and TRH.

Advanced Insights

  • T4 serves as a prohormone for T3.

  • Divergence in thyroid hormone actions related to cellular energy metabolism and regulatory factor expressions.

Conclusion

  • Summary of thyroid hormone biosynthesis, hormonal control, and physiological impacts.

  • Overview provides groundwork for understanding thyroid hormones' crucial roles in human physiology and health.