Anti-thyroid drugs: mechanisms and pharmacotherapy MCQs With Answer

This set of 20 advanced MCQs focuses on the mechanisms and pharmacotherapy of anti-thyroid drugs, tailored for M.Pharm students. Questions cover molecular mechanisms (thyroid peroxidase inhibition, peripheral deiodinase blockade), pharmacokinetics, clinical uses (graves’, toxic nodular goiter, thyroid storm), preoperative and pregnancy considerations, adverse effects (agranulocytosis, hepatotoxicity, teratogenicity), iodine physiology (Wolff–Chaikoff, Jod-Basedow), and therapeutic monitoring. Each item integrates mechanistic insight and clinical application to develop problem-solving skills essential for advanced pharmacology. Answers are provided to reinforce learning and highlight nuances important for safe and effective antithyroid therapy and patient counseling.

Q1. Which of the following best describes the primary molecular action of methimazole on thyroid hormone synthesis?

• Inhibition of 5′-deiodinase in peripheral tissues
• Competitive antagonism at TSH receptors
• Inhibition of thyroid peroxidase-catalyzed iodination and coupling
• Blockade of thyroglobulin endocytosis and hormone release

Correct Answer: Inhibition of thyroid peroxidase-catalyzed iodination and coupling

Q2. Propylthiouracil (PTU) differs from methimazole by which clinically important additional action?

• Stimulates thyroid hormone receptor degradation
• Inhibits peripheral conversion of T4 to T3 (5′-deiodinase)
• Enhances iodine uptake into follicular cells
• Precipitates immune-mediated thyroiditis

Correct Answer: Inhibits peripheral conversion of T4 to T3 (5′-deiodinase)

Q3. Carbimazole is used clinically because it is:

• A potent inhibitor of 5′-deiodinase with long half-life
• A prodrug that is converted to methimazole in vivo
• Less likely to cause agranulocytosis than methimazole
• Safest anti-thyroid drug for use throughout pregnancy

Correct Answer: A prodrug that is converted to methimazole in vivo

Q4. A patient with severe hyperthyroidism requires rapid reduction of thyroid hormone release before thyroidectomy. Which regimen is most appropriate preoperatively?

• High-dose methimazole alone for 24 hours
• Radioactive iodine 131 the day before surgery
• Lugol’s iodine (potassium iodide) plus a thionamide for 7–10 days
• Oral levothyroxine to suppress TSH

Correct Answer: Lugol’s iodine (potassium iodide) plus a thionamide for 7–10 days

Q5. Which adverse effect is classically associated with antithyroid thionamides and requires immediate drug discontinuation and CBC evaluation?

• Rash and mild fever
• Agranulocytosis with severe neutropenia
• Transient hypothyroidism
• Hyperprolactinemia

Correct Answer: Agranulocytosis with severe neutropenia

Q6. Which statement about radioactive iodine (I-131) therapy for hyperthyroidism is true?

• It is contraindicated in toxic nodular goiter
• It acts by inhibiting thyroid peroxidase enzymatically
• It destroys thyroid tissue via beta-emission causing gradual hypothyroidism
• It is the treatment of choice in pregnancy

Correct Answer: It destroys thyroid tissue via beta-emission causing gradual hypothyroidism

Q7. The Wolff–Chaikoff effect explains which phenomenon related to iodine exposure?

• Iodine always increases thyroid hormone synthesis (Jod-Basedow)
• Acute high iodine transiently suppresses thyroid hormone synthesis
• Iodine permanently inactivates thyroid peroxidase
• Chronic iodine deficiency prevents goiter formation

Correct Answer: Acute high iodine transiently suppresses thyroid hormone synthesis

Q8. In the management of thyroid storm, which combination addresses both hormone synthesis and peripheral effects most effectively?

• Methimazole alone
• Propranolol, PTU, potassium iodide, and glucocorticoids
• Levothyroxine and iodine supplementation
• Radioactive iodine and NSAIDs

Correct Answer: Propranolol, PTU, potassium iodide, and glucocorticoids

Q9. Methimazole is generally preferred over PTU except in which circumstance?

• First trimester of pregnancy
• Toxic multinodular goiter in elderly patients
• Mild subclinical hyperthyroidism
• Patients with agranulocytosis history

Correct Answer: First trimester of pregnancy

Q10. Which laboratory change is expected earliest after initiating thionamide therapy for hyperthyroidism?

• Rapid decrease in serum TSH within 24 hours
• Immediate fall in serum T3/T4 levels within hours
• Gradual decline in serum T3/T4 over weeks due to blocking new synthesis
• Increase in thyroglobulin within 48 hours

Correct Answer: Gradual decline in serum T3/T4 over weeks due to blocking new synthesis

Q11. Which mechanism explains how beta-blockers like propranolol provide benefit in thyrotoxicosis beyond symptom control?

• Stimulate thyroid peroxidase activity
• Directly bind and inhibit TSH receptors
• Reduce peripheral conversion of T4 to T3 at high doses
• Increase renal clearance of thyroid hormones

Correct Answer: Reduce peripheral conversion of T4 to T3 at high doses

Q12. Which adverse hepatic reaction is more strongly associated with PTU and influences prescribing practices?

• Cholestatic jaundice with eosinophilia
• Severe fulminant hepatic failure in rare cases
• Isolated mild transaminitis without consequence
• Hepatic steatosis reversible on drug cessation

Correct Answer: Severe fulminant hepatic failure in rare cases

Q13. Which scenario best represents the Jod-Basedow phenomenon?

• Sudden hypothyroidism after high iodine exposure in normal thyroid
• Exacerbation of thyrotoxicosis after iodine administration in autonomous thyroid nodules
• Iodine-induced remission of Graves’ disease
• Permanent suppression of thyroid hormone synthesis after brief iodide therapy

Correct Answer: Exacerbation of thyrotoxicosis after iodine administration in autonomous thyroid nodules

Q14. Monitoring for agranulocytosis during thionamide therapy primarily requires which patient instruction?

• Report fever, sore throat or mouth ulcers immediately
• Check daily TSH at home
• Monitor ECG weekly for QT prolongation
• Measure urinary iodine weekly

Correct Answer: Report fever, sore throat or mouth ulcers immediately

Q15. In a breastfeeding mother with hyperthyroidism, which statement guides antithyroid drug choice?

• Methimazole is absolutely contraindicated during lactation
• PTU is preferred during lactation due to lower milk transfer
• Radioactive iodine is safe while breastfeeding
• No antithyroid therapy should be given to breastfeeding mothers

Correct Answer: PTU is preferred during lactation due to lower milk transfer

Q16. Which pharmacokinetic characteristic is true for methimazole compared with PTU?

• Methimazole has a much shorter plasma half-life than PTU
• Methimazole requires multiple daily dosing while PTU is once daily
• Methimazole is well absorbed orally and has a longer half-life permitting once-daily dosing
• Methimazole is not orally bioavailable and must be IV

Correct Answer: Methimazole is well absorbed orally and has a longer half-life permitting once-daily dosing

Q17. Which immune mechanism underlies many cases of Graves’ disease targeted by antithyroid therapy?

• TSH receptor-stimulating autoantibodies (TSI) that activate thyroid hormone synthesis
• Autoantibodies causing complement-mediated follicular cell lysis
• Cytotoxic T-cell destruction leading to hypothyroidism
• B cell depletion due to methimazole therapy

Correct Answer: TSH receptor-stimulating autoantibodies (TSI) that activate thyroid hormone synthesis

Q18. Which treatment is least appropriate for a young woman with large toxic multinodular goiter seeking definitive therapy but who wishes future pregnancy?

• Total thyroidectomy after euthyroid preparation
• Radioactive iodine therapy prior to conception
• Long-term methimazole therapy to control hyperthyroidism
• Subtotal thyroidectomy with appropriate counseling

Correct Answer: Radioactive iodine therapy prior to conception

Q19. Which laboratory pattern suggests effective control of hyperthyroidism with antithyroid drugs but risk of developing hypothyroidism if therapy continued?

• Low TSH, high free T4
• Normal TSH, normal free T4 and T3
• High TSH, low free T4
• Undetectable TSH with low T3/T4

Correct Answer: High TSH, low free T4

Q20. Which drug interaction is clinically relevant when a patient on methimazole is also prescribed warfarin?

• Methimazole causes marked CYP induction increasing warfarin clearance
• Methimazole potentiates warfarin effect by increasing warfarin metabolism
• Thyroid status changes during treatment can alter warfarin sensitivity and require INR monitoring
• No interaction; warfarin dosing remains stable during changes in thyroid status

Correct Answer: Thyroid status changes during treatment can alter warfarin sensitivity and require INR monitoring

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