Thyroid hormones

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Thyroid hormones


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Thyroid hormones

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USMLE® Step 1 style questions USMLE

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An essential step in the formation of thyroid hormones is the organification of iodine to form monoiodothyronine (MIT) and di-iodothyronine (DIT). Which of the following molecules is responsible for this reaction?  

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thyroid hormones and p. 362


thyroid hormones for p. 362


thyroid hormones p. 362

Thyroid hormones p. 342

signaling pathways for p. 353

in toxic multinodular goiter p. 348


Thyroid hormones are triiodothyronine or T3, and thyroxine, also called T4 or tetraiodothyronine.

They’re two tyrosine-based iodine-containing hormones that help regulate our body’s metabolism.

T3 and T4 are produced by the thyroid gland, which is located in the neck and consists of two lobes that look like two thumbs hooked together in the shape of a “V”.

If we zoom into the thyroid gland, we’ll find thousands of follicles, which are small hollow spheres whose walls are lined with follicular cells or thyrocytes, and are separated by a small amount of connective tissue.

Follicular cells are bipolar cells - with an apical side that surrounds a central cavity or lumen filled with the colloid, which is a fluid that contains the precursor hormone thyroglobulin.

Thyroglobulin is a large glycoprotein that is synthesized by the follicular cells and then it is secreted into the lumen of the follicle to be stored in the colloid.

The basolateral side of follicular cells is in contact with blood vessels.

The basolateral membrane contains a sodium-iodide symporter, which pumps two sodium ions into the cell down their electrochemical gradient, in order to bring one iodide ion into the cell from blood.

Iodide is then pumped into the colloid through an ion transporter called pendrin, which exchanges iodide for chloride, since they’re both negatively charged.

Once iodide is in the colloid, it undergoes oxidation with the enzyme thyroperoxidase, which changes it into an iodine atom.

It’s then attached to tyrosine amino acid residues which are found throughout thyroglobulin.

This process is called iodination.

Some tyrosine residues are bound by only one iodine, whereas others are bound by two iodine atoms, yielding monoiodotyrosine or MIT, and diiodotyrosine or DIT, respectively.

These molecules are then linked together by thyroperoxidase.

Linking one MIT with one DIT creates T3, while linking two DIT molecules creates T4 - and both T3 and T4 remain bound to thyroglobulin.

You see - thyroglobulin basically serves as a peptide that stores these hormones on it in the colloid, until it’s ready to be used. T4 is created in greater amounts than T3.

When the follicular cells are ready to secrete T3 and T4, they endocytose the thyroglobulin into a vesicle which then fuses with the lysosome.

Here, thyroglobulin is cleaved by proteases, and T3 and T4 are released right into the bloodstream through the monocarboxylate or MCT transporter.

T3 is the highly active form with a half life of one to two days, while T4 is the less active form with a longer half life of six to eight days.


Thyroid hormones are hormones produced by the thyroid gland, which is located in the neck. The two thyroid hormones are thyroxine (T4) and triiodothyronine (T3); with T3 having the more active role in metabolic activities. Both thyroxine (T4) and triiodothyronine (T3) are produced and secreted by the thyroid gland in response to the thyroid-stimulating hormone which is secreted by the anterior pituitary lobe.

Thyroid hormones play a vital role in regulating metabolism and controlling the rate at which the body burns calories and uses energy. They also help to regulate body temperature, heart rate, and blood pressure, as well as growth and development in children.


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  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "Thyroid Hormone Regulation of Metabolism" Physiological Reviews (2014)
  6. "Plasma Membrane Transport of Thyroid Hormones and Its Role in Thyroid Hormone Metabolism and Bioavailability" Endocrine Reviews (2001)
  7. "Biochemistry, Cellular and Molecular Biology, and Physiological Roles of the Iodothyronine Selenodeiodinases" Endocrine Reviews (2002)
  8. "undefined" Reviews in Endocrine and Metabolic Disorders (2000)

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