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Thyroid Hormone Production Pathway and Systemic Effects

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Thyroid Hormone Synthesis Pathway

  1. Thyroglobulin (TG) Synthesis and Transport

    TG synthesis occurs in the Rough Endoplasmic Reticulum (RER) and Golgi apparatus. The synthesized TG is then transported to the follicular lumen.

  2. Iodide Uptake (Iodide Pump)

    Iodide (I-) enters the follicular epithelial cells via the sodium-iodide symporter (NIS) pump. Inhibitors such as perchlorate and thiocyanate block the uptake of iodide into the follicular cells, thereby interfering with thyroid hormone synthesis.

  3. Oxidation of Iodide (I- to I2)

    Iodide crosses the apical membrane where it is oxidized to form molecular iodine (I2). The enzyme Thyroid Peroxidase (TPO) catalyzes this oxidation in two steps. Propylthiouracil (PTU) inhibits TPO, blocking hormone synthesis.

  4. Organification of Iodine (MIT and DIT Formation)

    At the apical membrane, I2 combines with tyrosine residues on the Thyroglobulin (TG) molecule to form monoiodotyrosine (MIT) and diiodotyrosine (DIT).

  5. Coupling Reaction (T3 and T4 Synthesis)

    This reaction is also catalyzed by Thyroid Peroxidase (TPO):

    • Two molecules of DIT combine to form Thyroxine (T4). (This reaction is faster and produces a greater quantity.)
    • One molecule of DIT combines with one molecule of MIT to form Triiodothyronine (T3).

    Uncoupled MIT and DIT residues remain attached to the TG molecule.

  6. TG Endocytosis

    Thyroglobulin containing T3 and T4 is taken back into the follicular cell via endocytosis at the apical membrane.

  7. Hydrolysis and Release

    Lysosomal enzymes hydrolyze the TG molecule, releasing free T3 and T4. These hormones then enter the systemic circulation.

  8. Degradation and Recycling

    Unused MIT and DIT are degraded intracellularly by the enzyme iodotyrosine deiodinase (or dehalogenase), recycling iodide and tyrosine for reuse in hormone synthesis.

Physiological Effects of Thyroid Hormones (T3/T4)

Metabolic Effects

  • Increases intestinal glucose absorption.
  • Increases glucose synthesis from glycogen (glycogenolysis).
  • Increases the release of free fatty acids from adipocytes (lipolysis).
  • Causes protein degradation (proteolytic effect).
  • Stimulates glycogenesis (synthesis of glycogen).

Growth and Maturation

Thyroid hormones are crucial for normal growth and development:

  • Stimulate ossification and linear growth of bones.
  • Promote epiphyseal maturation and the activity of chondrocytes in cartilaginous plates.
  • Support the production and action of somatomedins (IGF-1).

Central Nervous System (CNS) Development

Thyroid hormones are essential for fetal and neonatal brain development:

  • Stimulate the growth of the cerebral cortex and cerebellum.
  • Stimulate the proliferation of axons and dendritic arborizations.
  • Stimulate synaptogenesis (formation of synapses).
  • Stimulate myelination.
  • Stimulate neuronal cell migration.

Thermoregulation

Thyroid hormones are calorigenic hormones, meaning they increase the basal metabolic rate and oxygen consumption in most tissues, thereby helping to maintain normal body temperature.

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