5.1 Thyroid Hormone Transport & Conversion

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5.1 Thyroid Hormone Transport & Conversion — Test 1

Q1. In the circulation, most T3 and T4 are carried bound to:✓ Thyroxine-binding globulin (TBG)

Q2. The biologically active fraction of circulating thyroid hormone is the:✓ Free (unbound) hormone

Q3. In pregnancy, TBG levels rise. The expected pattern is:✓ Increased total T4 with normal free T4 (euthyroid)

Q4. In hepatic failure, TBG falls. This causes:✓ Decreased total T4 with normal free T4

Q5. Peripheral conversion of T4 to the more active T3 is catalysed by:✓ 5'-deiodinase (outer-ring)

Q6. Reverse T3 (rT3) is:✓ Metabolically inactive and increased in starvation/illness

Q7. T3 is more potent than T4 and target tissues largely generate T3 by:✓ Local deiodination of T4

Q8. Match each state with its effect on thyroid-hormone binding and choose the correct option.✓ A-ii, B-iii, C-i

Q9. Why might a patient with abnormal TBG levels still be clinically euthyroid?✓ Free hormone levels remain normal despite altered total hormone

Q10. Propylthiouracil, besides blocking synthesis, also lowers active hormone by inhibiting:✓ Peripheral 5'-deiodinase (T4 -> T3 conversion)

Q11. The half-life of T4 is approximately a week, mainly because it is:✓ Strongly protein-bound (to TBG)

Q12. In the sick-euthyroid (non-thyroidal illness) state, a typical change is:✓ Increased rT3 and decreased T3

Q13. Drugs or states that RAISE TBG (e.g. estrogen/pregnancy) will:✓ Raise total T4 but keep free T4 normal

Q14. The main circulating form measured to assess thyroid hormone reserves (though inactive) is:✓ Total/free T4

Q15. A patient with high total T4 but normal TSH and free T4 most likely has:✓ Increased TBG (e.g. pregnancy/estrogen)

Q16. Inner-ring (5-) deiodination of T4 produces:✓ Reverse T3 (inactive)

Q17. Compared with T4, T3 in the blood is:✓ Less protein-bound and has a shorter half-life

Q18. The deiodinase enzymes responsible for activation/inactivation of thyroid hormone require the trace element:✓ Selenium

Q19. Why is T4 often described as a 'prohormone'?✓ It is converted peripherally to the more active T3

Q20. A rise in free T4 with a suppressed TSH indicates:✓ Hyperthyroidism (true hormone excess)