Antithyroid drugs MCQs With Answer is a focused review designed for B. Pharm students to master pharmacology of hyperthyroidism. This concise, exam-oriented introduction covers mechanisms, pharmacokinetics, clinical uses, pregnancy and pediatric considerations, adverse effects (agranulocytosis, hepatotoxicity, vasculitis), monitoring, drug interactions, iodide therapy, radioiodine and surgical indications. Key drugs discussed include methimazole, propylthiouracil (PTU), carbimazole, and adjunctive agents such as iodides and beta-blockers. Emphasis on mechanisms (TPO inhibition, peripheral T4→T3 conversion), safety in pregnancy, and laboratory monitoring prepares students for clinical decision-making and exams. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. Which primary mechanism explains how methimazole and propylthiouracil reduce thyroid hormone synthesis?
- Inhibition of thyroid peroxidase (TPO)
- Blockade of the sodium-iodide symporter (NIS)
- Antagonism of thyrotropin-releasing hormone (TRH) receptors
- Inhibition of TSH secretion from the pituitary
Correct Answer: Inhibition of thyroid peroxidase (TPO)
Q2. Which antithyroid drug also inhibits peripheral conversion of T4 to the more active T3?
- Methimazole
- Propylthiouracil (PTU)
- Carbimazole
- Potassium iodide
Correct Answer: Propylthiouracil (PTU)
Q3. Which antithyroid agent is preferred during the first trimester of pregnancy to reduce teratogenic risk?
- Methimazole
- Propylthiouracil (PTU)
- Carbimazole
- Radioactive iodine
Correct Answer: Propylthiouracil (PTU)
Q4. Which serious hematologic adverse effect requires immediate drug discontinuation and urgent evaluation in patients on thionamides?
- Agranulocytosis (severe neutropenia)
- Hemolytic anemia
- Thrombocytosis
- Polycythemia
Correct Answer: Agranulocytosis (severe neutropenia)
Q5. Which antithyroid drug is most associated with severe, rare fulminant hepatic failure?
- Methimazole
- Propylthiouracil (PTU)
- Carbimazole
- Potassium iodide
Correct Answer: Propylthiouracil (PTU)
Q6. What is the typical clinical onset of symptomatic improvement after starting antithyroid drugs?
- Immediate (within hours)
- 1–3 days
- 2–4 weeks
- 6–12 months
Correct Answer: 2–4 weeks
Q7. What does a “block-and-replace” regimen mean in antithyroid therapy?
- Blocking dietary iodine and replacing with supplements
- Using antithyroid drug to block hormone synthesis then giving levothyroxine replacement
- Replacing methimazole with PTU after initial blockade
- Using radioactive iodine followed by thyroid hormone replacement
Correct Answer: Using antithyroid drug to block hormone synthesis then giving levothyroxine replacement
Q8. Which laboratory parameter is most useful for short-term monitoring of response to antithyroid therapy?
- TSH
- Free T4
- Total cholesterol
- Serum calcium
Correct Answer: Free T4
Q9. Methimazole is associated with which specific teratogenic risk if used in early pregnancy?
- Cardiac teratogenesis
- Aplasia cutis and scalp defects
- Limb defects similar to thalidomide
- Neural tube defects
Correct Answer: Aplasia cutis and scalp defects
Q10. What is the mechanism by which radioactive iodine (I-131) treats hyperthyroidism?
- Competitive blockade of TSH receptors
- Destruction of thyroid follicular cells by beta radiation
- Inhibition of peripheral T4 to T3 conversion
- Stimulation of thyroid autoantibodies to suppress function
Correct Answer: Destruction of thyroid follicular cells by beta radiation
Q11. Why is saturated potassium iodide or Lugol’s solution given preoperatively for Graves’ disease?
- To increase thyroid hormone synthesis before surgery
- To reduce thyroid vascularity and decrease hormone release (Wolff–Chaikoff effect)
- To stimulate TSH release and shrink the gland
- To permanently destroy thyroid tissue
Correct Answer: To reduce thyroid vascularity and decrease hormone release (Wolff–Chaikoff effect)
Q12. Which laboratory finding is diagnostic for agranulocytosis in a patient on antithyroid therapy?
- Platelet count <50,000/µL
- Absolute neutrophil count <500 cells/µL
- Hemoglobin <8 g/dL
- Lymphocyte percentage >50%
Correct Answer: Absolute neutrophil count <500 cells/µL
Q13. Which agent is most useful initially for rapid symptomatic control of tachycardia and tremor in thyroid storm?
- Propranolol (beta-blocker)
- Methimazole
- Potassium iodide
- Radioactive iodine
Correct Answer: Propranolol (beta-blocker)
Q14. What is the most accepted mechanism for thionamide-induced agranulocytosis?
- Direct marrow toxicity unrelated to immune response
- Immune-mediated destruction of neutrophils or marrow precursors
- Excess iodine causing marrow suppression
- Hypersensitivity leading to eosinophilia
Correct Answer: Immune-mediated destruction of neutrophils or marrow precursors
Q15. Starting methimazole therapy in a patient on warfarin typically requires what change in warfarin dosing?
- Increase warfarin dose substantially
- No change in warfarin dose
- Decrease warfarin dose
- Switch warfarin to a direct oral anticoagulant immediately
Correct Answer: Decrease warfarin dose
Q16. Which statement correctly compares methimazole and PTU for pregnancy and overall safety?
- Methimazole is preferred in the first trimester, PTU thereafter
- PTU is preferred in the first trimester; methimazole preferred after the first trimester due to lower hepatotoxicity risk
- Both drugs are always contraindicated in pregnancy
- Methimazole has no teratogenic risks and is safe throughout pregnancy
Correct Answer: PTU is preferred in the first trimester; methimazole preferred after the first trimester due to lower hepatotoxicity risk
Q17. After initiating antithyroid medication, when should thyroid function tests typically be rechecked to assess response?
- 24–48 hours
- 1 week
- 4–6 weeks
- 12 months
Correct Answer: 4–6 weeks
Q18. The Wolff–Chaikoff effect describes which action of high-dose iodide therapy?
- Permanently increasing thyroid hormone synthesis
- Inhibiting organification of iodide and transiently blocking hormone release
- Stimulating TSH secretion to reduce gland size
- Enhancing peripheral T4 to T3 conversion
Correct Answer: Inhibiting organification of iodide and transiently blocking hormone release
Q19. Which autoantibody is most closely associated with Graves’ disease and stimulates thyroid hormone production?
- Anti-thyroid peroxidase (anti-TPO) antibodies
- TSH receptor-stimulating immunoglobulins (TSI)
- Anti-thyroglobulin antibodies
- Anti-insulin antibodies
Correct Answer: TSH receptor-stimulating immunoglobulins (TSI)
Q20. Which is considered a definitive long-term therapy for Graves’ hyperthyroidism in adults?
- Short course of potassium iodide alone
- Long-term propranolol monotherapy
- Radioactive iodine ablation
- Intermittent corticosteroid therapy
Correct Answer: Radioactive iodine ablation
Q21. Which agent is a prodrug that is converted to methimazole in the body?
- Carbimazole
- Propylthiouracil (PTU)
- Perchlorate
- Levothyroxine
Correct Answer: Carbimazole
Q22. Which adverse effect has been particularly associated with propylthiouracil and is often ANCA-positive?
- Interstitial nephritis
- ANCA-positive vasculitis
- Myocarditis
- Hypokalemia
Correct Answer: ANCA-positive vasculitis
Q23. Which antithyroid drug has a shorter plasma half-life necessitating more frequent daily dosing?
- Methimazole (longer half-life)
- Propylthiouracil (PTU)
- Carbimazole (very long half-life)
- Radioactive iodine
Correct Answer: Propylthiouracil (PTU)
Q24. What is the primary goal of antithyroid therapy prior to elective thyroid surgery?
- Induce permanent hypothyroidism before surgery
- Render the patient euthyroid and reduce surgical risk
- Increase thyroid gland vascularity to facilitate resection
- Stimulate autoimmune destruction of the gland
Correct Answer: Render the patient euthyroid and reduce surgical risk
Q25. Which antithyroid drug is generally preferred for long-term medical management of hyperthyroidism in children?
- Propylthiouracil (PTU)
- Methimazole
- Radioactive iodine at high dose
- Amiodarone
Correct Answer: Methimazole
Q26. Which pattern of thyroid function tests indicates overtreatment with antithyroid drugs leading to hypothyroidism?
- Low TSH, high free T4
- High TSH, low free T4
- Normal TSH, high free T4
- High TSH, high free T4
Correct Answer: High TSH, low free T4
Q27. Maternal use of methimazole during pregnancy can cause which fetal thyroid abnormality?
- Fetal hyperthyroidism only
- Fetal hypothyroidism and goiter
- No effect on the fetal thyroid
- Permanent neonatal hyperparathyroidism
Correct Answer: Fetal hypothyroidism and goiter
Q28. Which radioactive iodine uptake pattern on a thyroid scan is most consistent with Graves’ disease?
- Poor uptake diffusely
- Patchy focal uptake in nodules
- Diffuse increased uptake throughout the gland
- Uptake only in the isthmus
Correct Answer: Diffuse increased uptake throughout the gland
Q29. Carbimazole exerts its antithyroid effect primarily because:
- It directly blocks peripheral T3 receptors
- It is converted to methimazole, which inhibits TPO
- It chelates iodine in the circulation
- It stimulates TSH release to suppress the gland
Correct Answer: It is converted to methimazole, which inhibits TPO
Q30. What is the approximate relapse rate after a standard 12–18 month course of antithyroid drug therapy for Graves’ disease?
- Very low (<5%)
- Moderate (10–20%)
- High (~40–60%)
- Essentially zero after stopping therapy
Correct Answer: High (~40–60%)
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