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1.3 Thyroid Hormone Synthesis — Test 1
Q1. Thyroid hormones are synthesized on the scaffold of a large protein secreted into the colloid, namely:✓ Thyroglobulin
Q2. Iodide is actively concentrated in thyroid follicular cells by the:✓ Na+/I- symporter (NIS)
Q3. The iodide pump (NIS) of the thyroid is inhibited by:✓ Thiocyanate and perchlorate
Q4. Oxidation of trapped iodide to its reactive form, and the subsequent iodination/coupling, are catalysed by:✓ Thyroid peroxidase
Q5. Propylthiouracil (PTU) reduces thyroid hormone synthesis by inhibiting:✓ Thyroid peroxidase (oxidation/organification/coupling)
Q6. Organification in thyroid hormone synthesis refers to:✓ Iodination of tyrosine residues on thyroglobulin to form MIT and DIT
Q7. During coupling on thyroglobulin, thyroxine (T4) is formed by the combination of:✓ DIT + DIT
Q8. High circulating iodide transiently inhibits organification and thyroid hormone synthesis. This is the:✓ Wolff-Chaikoff effect
Q9. Most of an injected dose of radioactive iodine in a mammal accumulates in the:✓ Thyroid
Q10. After TSH stimulation, iodinated thyroglobulin is taken back into the follicular cell and T3/T4 are released by:✓ Endocytosis (pinocytosis) followed by lysosomal proteolysis
Q11. In peripheral tissues, the conversion of T4 to the more active T3 is carried out by:✓ 5'-deiodinase
Q12. Reverse T3 (rT3) is best described as:✓ A metabolically inactive form, increased in starvation
Q13. In the circulation, most T3 and T4 are carried bound to:✓ Thyroxine-binding globulin (TBG)
Q14. Each step in thyroid hormone synthesis and secretion is stimulated by:✓ TSH (thyroid-stimulating hormone)
Q15. Match each agent with its action in thyroid physiology and choose the correct option.✓ A-iii, B-i, C-ii
Q16. Leftover MIT and DIT (not coupled into hormone) are recycled by thyroid deiodinase. A deficiency of this enzyme would:✓ Mimic iodine deficiency (iodide is lost, not reused)
Q17. Dietary iodine deficiency leads to goitre because low thyroid hormone causes:✓ Loss of feedback inhibition, raising TRH and TSH
Q18. Which form of iodine is used to iodinate (organify) the tyrosine residues of thyroglobulin?✓ Oxidized iodine
Q19. More of which hormone is synthesized by the thyroid gland, even though the other is more potent?✓ More T4 is made; T3 is more potent
Q20. Pendrin, located on the apical (colloid-facing) membrane of the thyroid follicular cell, functions to:✓ Transport iodide toward the colloid for organification