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Physiology I
Section 6
Thyroid Hormones

Recommended Reading: Guyton ch. 76  

Key Words

Follicle cell-these cells surround the colloid containing lumen of the thyroid follicle.
These cells are cuboidal when they are inactive. When they are active they elongate and become columnar in shape. They are stimulated by Thyroid Stimulating Hormone (TSH). These cells play a role in the production of T3 and T4.
*See Handout from 8/19 for figures and additional info.

Iodide- from diet is transferred across the basement membrane of the follicle cell (from the blood to the cytosol) and becomes oxidized (from 2I- to I2). This reaction is catalyzed by Thyroid Peroxidase (TPO). Tyrosine molecules of the thyroglobulin protein are iodized on the tyrosine amino acids to produce monoiodotyrosine(MIT) and diiodotyrosine(DIT). This reaction is also catalyzed by TPO. MIT=T1, DIT=T2 These molecules are coupled with each other to produce T3 and T4. MIT+DIT--->T3 and DIT+DIT--->T4 Iodide is later removed from these hormones (by deiodizing enzymes) and recycled.

Thyroglobulin- high molecular weight glycoprotein containing ~ 100 tyrosine which can become iodized. They are produced by the Rough Endoplasmic Reticulum and modified in the Golgi Apparatus. They are packaged into small vesicles and sent to the lumen of the thyroid by exocytosis.

MIT- monoiodotyrosine/T1 This is created when tyrosine is iodized once(TPO). This will couple with DIT to become T3.

DIT- diiodotyrosine/T2 This is created when tyrosine is iodized twice (TPO). Can couple with MIT to become T3 or more commonly it can couple with another DIT to become T4.

T3- triiodothyronine, thyroid hormone which contains 3 iodide atoms. It is made from MIT + DIT, or by T4 cleaved by 5ímonodeiodinase. It is the ACTIVE thyroid hormone. T3 is more biologically active and has a higher affinity for its receptor protein at the sight of action. * see funcns below

T4- thyroxine, thyroid hormone which contains 4 iodide atoms. It is made from two DITs. Can be cleaved by 5ímonodeiodinase to become T3 which is more potent. T4 is created in a much larger quantity than T3. It is moved from the thyroid and has a longer 1/2 life than T3 (7 days vs. 1.3 days), has a very high affinity for TBG * see functions below

*(T3 and T4) Thyroid hormones have 3 major functions: Regulate O2 use and Basal Metabolic Rate, Regulate Cellular Metabolism, and Regulate Growth and Development

Thyroxine Binding Globulin (TBG)- binding protein which binds to T3 and T4 in blood. TBG+T3 have a half life of 1.3days, TBG+T4 have a half life of 7 days. Due to the high affinity for TBG, T3 and T4 are released into tissue cells slowly. Once they are inside they bind to intracellular proteins.



Thyroid-stimulating antibodies:  

Graves' Disease:  

Hashimoto's Thyroiditis:  


Total vs. Free thyroid hormone measurement:  


1.  Describe thyroid hormone syntheses and storage, and thyroid gland compartmentalization.


  1. Iodide Trapping-Iodide (I-) is actively transported across the basement membrane of the follicle cell and trapped.
  2. Oxidation of Iodide- I- + I- -->I2 (oxidation by TPO)
  3. Synthesis of TBG- thyroglobulin glycoprotein is made and extruded into lumen of follicle
  4. Iodination of Tyrosine-Tyrosines bind I2 to become MIT, DIT (TPO)
  5. Coupling of T1 and T2 (or MIT and DIT)- within Thyroglobulin these molecules couple to produce T3 (MIT+DIT) and T4(DIT+DIT) This step is also catalyzed by TPO.
  6. Pinocytosis and Digestion of Colloid- Thyroglobulin reenters the cell by endocytosis and is hydrolyzed
  7. Deiodination of MIT, DIT to reuse Iodide
  8. Transport of T3, T4 through blood to target tissues. T3 and T4 are bound to TBG when they are in blood.



2.  Discuss the regulation of release, hormonal transport, metabolism, target tissues, and physiological effects of thyroid hormone:  

3.  How do T3 and T4 differ in their structure, half-life and biologic activity?:  

4.  What is the difference between a total and free thyroid hormone measurement?: 


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