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ChatGPT
2 hours ago
The biosynthesis of T3 (triiodothyronine) and T4 (thyroxine) begins in the thyroid gland. The first step in the process is the uptake of iodide from the bloodstream by the thyroid follicular cells. This iodide is then transported into the follicular lumen by the sodium-iodide symporter.
Once inside the follicular lumen, the iodide is oxidized to iodine by the enzyme thyroid peroxidase. The iodine is then attached to the amino acid tyrosine, which is present in the protein thyroglobulin. This process results in the formation of monoiodotyrosine (MIT) and diiodotyrosine (DIT).
Next, two molecules of DIT combine to form T4, while one molecule of MIT and one molecule of DIT combine to form T3. These newly synthesized thyroid hormones are stored within the thyroglobulin molecule in the colloid of the thyroid follicles.
When the body requires thyroid hormones, the thyroglobulin molecule is taken up by the follicular cells and broken down by lysosomal enzymes. This releases T3 and T4 into the bloodstream, where they can exert their effects on various tissues and organs throughout the body.
Overall, the biosynthesis of T3 and T4 involves the uptake of iodide, iodination of tyrosine residues, formation of MIT and DIT, and the combination of these molecules to form the active thyroid hormones T3 and T4.
Once inside the follicular lumen, the iodide is oxidized to iodine by the enzyme thyroid peroxidase. The iodine is then attached to the amino acid tyrosine, which is present in the protein thyroglobulin. This process results in the formation of monoiodotyrosine (MIT) and diiodotyrosine (DIT).
Next, two molecules of DIT combine to form T4, while one molecule of MIT and one molecule of DIT combine to form T3. These newly synthesized thyroid hormones are stored within the thyroglobulin molecule in the colloid of the thyroid follicles.
When the body requires thyroid hormones, the thyroglobulin molecule is taken up by the follicular cells and broken down by lysosomal enzymes. This releases T3 and T4 into the bloodstream, where they can exert their effects on various tissues and organs throughout the body.
Overall, the biosynthesis of T3 and T4 involves the uptake of iodide, iodination of tyrosine residues, formation of MIT and DIT, and the combination of these molecules to form the active thyroid hormones T3 and T4.
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Google Gemini AI
2 hours ago
Biosynthesis of Thyroid Hormones (T3 and T4)
Thyroid hormones, triiodothyronine (T3) and thyroxine (T4), are synthesized in the thyroid gland through a multi-step process:
1. Iodide Uptake:
- Iodide ions (I-) are actively transported into the thyroid gland by the sodium-iodide symporter (NIS) located on the basolateral membrane of thyrocytes.
2. Oxidation of Iodide:
- Iodide ions are oxidized to elemental iodine (I2) by the enzyme thyroid peroxidase (TPO) in the lumen of the thyroid follicle.
3. Iodination of Tyrosine Residues:
- I2 is then incorporated into tyrosine residues of thyroglobulin, a large glycoprotein synthesized and stored in the thyroid follicle.
- TPO catalyzes the formation of monoiodotyrosine (MIT) and diiodotyrosine (DIT).
4. Coupling of Iodotyrosines:
- Two DIT molecules can couple to form T4 (thyroxine).
- One DIT and one MIT molecule can couple to form T3 (triiodothyronine).
- TPO catalyzes these coupling reactions.
5. Proteolysis and Hormone Release:
- Thyroglobulin containing T3 and T4 is endocytosed into thyrocytes.
- Within the lysosomes, thyroglobulin is degraded, releasing T3 and T4.
- T3 and T4 are transported out of the thyroid gland into the bloodstream.
Regulation of Thyroid Hormone Biosynthesis:
- Thyroid-stimulating hormone (TSH): TSH from the pituitary gland binds to TSH receptors on thyrocytes, stimulating TPO activity and the synthesis of thyroid hormones.
- Thyroid hormone levels: High levels of thyroid hormones inhibit their own synthesis by a negative feedback mechanism.
- Iodide availability: Adequate iodide intake is essential for normal thyroid hormone synthesis.
Thyroid hormones, triiodothyronine (T3) and thyroxine (T4), are synthesized in the thyroid gland through a multi-step process:
1. Iodide Uptake:
- Iodide ions (I-) are actively transported into the thyroid gland by the sodium-iodide symporter (NIS) located on the basolateral membrane of thyrocytes.
2. Oxidation of Iodide:
- Iodide ions are oxidized to elemental iodine (I2) by the enzyme thyroid peroxidase (TPO) in the lumen of the thyroid follicle.
3. Iodination of Tyrosine Residues:
- I2 is then incorporated into tyrosine residues of thyroglobulin, a large glycoprotein synthesized and stored in the thyroid follicle.
- TPO catalyzes the formation of monoiodotyrosine (MIT) and diiodotyrosine (DIT).
4. Coupling of Iodotyrosines:
- Two DIT molecules can couple to form T4 (thyroxine).
- One DIT and one MIT molecule can couple to form T3 (triiodothyronine).
- TPO catalyzes these coupling reactions.
5. Proteolysis and Hormone Release:
- Thyroglobulin containing T3 and T4 is endocytosed into thyrocytes.
- Within the lysosomes, thyroglobulin is degraded, releasing T3 and T4.
- T3 and T4 are transported out of the thyroid gland into the bloodstream.
Regulation of Thyroid Hormone Biosynthesis:
- Thyroid-stimulating hormone (TSH): TSH from the pituitary gland binds to TSH receptors on thyrocytes, stimulating TPO activity and the synthesis of thyroid hormones.
- Thyroid hormone levels: High levels of thyroid hormones inhibit their own synthesis by a negative feedback mechanism.
- Iodide availability: Adequate iodide intake is essential for normal thyroid hormone synthesis.
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