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Questions and Answers
What type of protein is secreted by C cells in the thyroid gland?
Calcitonin
What is the function of thyroglobulin (Tg) in the thyroid gland?
To store iodine and tyrosine for T4 and T3 synthesis
What stimulates the uptake of iodide from the bloodstream into the thyroid gland?
Thyroid-stimulating hormone (TSH)
What is the role of Na+ in iodide transport in the thyroid gland?
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What is the product of tyrosine modification in the synthesis of thyroid hormones?
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What is the function of thyroperoxidase (TPO) in the thyroid gland?
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What is the purpose of deiodinase enzymes in the thyroid gland?
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What is the recommended daily intake of iodine?
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What is the source of iodide in the bloodstream?
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What is the fate of iodinated thyroglobulin in the thyroid gland?
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Study Notes
Thyroglobulin and Iodination
- Thyroperoxidase (TPO) iodinates tyrosyl residues of thyroglobulin (from ER)
- Megalin binds to iodinated thyroglobulin, leading to endocytosis back into the cell
- TSH binding stimulates the endocytosis of the colloid
Synthesis and Secretion of Thyroid Hormones
- Vesicles containing iodinated thyroglobulin fuse with lysosomes, where lysosomal enzymes cleave T4 from thyroglobulin
- T4 exits into the bloodstream
- T4 is the primary secretory product of the thyroid gland, accounting for approximately 70-90 μg/day
- The total daily production rate of T3 is about 15-30 μg
- T3 is derived from 2 processes: 80% from de-iodination of T4 in peripheral tissues and 20% from direct thyroid secretion
Deiodinase Enzymes
- Three deiodinase enzymes (Type I, II, and III) convert T4 to the more active T3
- Type I is present in liver, kidney, thyroid, and pituitary (80% of T4 deiodination)
- Type II is present in CNS, pituitary, brown adipose tissue, and heart (predominantly intracellular)
- Type III is present in placenta and CNS
Thyroperoxidase (TPO)
- TPO catalyzes the oxidation of I- by H2O2 to a reactive iodine intermediate
- This intermediate then iodinates tyrosine or mono-iodotyrosine
- TPO is an epitope of autoantibodies in autoimmune thyroid diseases
- TSH upregulates TPO expression
Thyroglobulin Proteolysis and Hormone Release
- Proteolysis of thyroglobulin is essential for releasing T4 and T3
- Thyroglobulin is phagocytosed into follicular cells in the form of colloid droplets, which fuse with lysosomes to form phagolysosomes
- Thyroglobulin is then hydrolyzed to T4 and T3, which are secreted into the circulation
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Description
This quiz covers the process of thyroid hormone synthesis, from the iodination of tyrosyl residues to the secretion of T4 and T3 into the bloodstream. Learn about the role of TSH, megalin, and lysosomal enzymes in this process.
