Understanding L-Thyronine is essential for B.Pharm students studying endocrine pharmacology. L-Thyronine refers to thyroid hormone preparations and analogs such as levothyroxine (T4) and liothyronine (T3). This topic covers mechanism of action (nuclear thyroid receptors), pharmacokinetics (absorption, bioavailability, protein binding, half-life), therapeutic indications (hypothyroidism, myxedema, TSH suppression), dosing strategies, monitoring (TSH, free T4), drug interactions (calcium, iron, warfarin, enzyme inducers), adverse effects (arrhythmia, osteoporosis), and special situations (pregnancy, myxedema coma). Mastery of these concepts is vital for safe dispensing, counseling, and clinical decision-making. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. Which molecule is the primary prohormone that is converted peripherally into the active thyroid hormone?
- Triiodothyronine (T3)
- Thyroid stimulating hormone (TSH)
- Levothyroxine (L-T4)
- Reverse T3 (rT3)
Correct Answer: Levothyroxine (L-T4)
Q2. What is the principal mechanism of action of L-Thyronine (thyroid hormones)?
- Activation of membrane G-protein coupled receptors
- Binding to cytosolic enzymes and inhibiting glycolysis
- Direct interaction with mitochondrial ATP synthase
- Binding nuclear thyroid hormone receptors to regulate gene transcription
Correct Answer: Binding nuclear thyroid hormone receptors to regulate gene transcription
Q3. Which preparation is the synthetic form of T4 commonly used in replacement therapy?
- Liothyronine sodium
- Levothyroxine sodium
- Thyroglobulin
- Propylthiouracil
Correct Answer: Levothyroxine sodium
Q4. Compared with levothyroxine, liothyronine (T3) is characterized by:
- Longer plasma half-life and slower onset
- Shorter half-life and more rapid onset of action
- Greater protein binding and reduced potency
- Exclusive use in pregnancy over levothyroxine
Correct Answer: Shorter half-life and more rapid onset of action
Q5. A major peripheral conversion step of thyroid hormones is:
- Deiodination of T4 to T3 by deiodinases
- Methylation of T3 to T4 in the liver
- Oxidation of rT3 to active T3
- Conjugation with glucuronic acid
Correct Answer: Deiodination of T4 to T3 by deiodinases
Q6. The oral bioavailability of levothyroxine in fasting adults is approximately:
- 10–20%
- 40–50%
- 70–80%
- 95–100%
Correct Answer: 70–80%
Q7. Which clinical parameter is most commonly used to monitor adequacy of levothyroxine replacement?
- Serum total T3
- Serum TSH
- Serum reverse T3
- 24-hour urinary iodine
Correct Answer: Serum TSH
Q8. Which drug class is known to decrease oral absorption of levothyroxine when co-administered?
- Calcium supplements and antacids
- Beta blockers
- ACE inhibitors
- SSRIs
Correct Answer: Calcium supplements and antacids
Q9. Estrogen therapy affects thyroid hormone requirements by:
- Decreasing TBG and reducing levothyroxine dose needed
- Increasing TBG and often increasing levothyroxine dose needed
- Blocking peripheral deiodinases and eliminating need for levothyroxine
- Directly inhibiting levothyroxine absorption in the gut
Correct Answer: Increasing TBG and often increasing levothyroxine dose needed
Q10. In elderly patients starting levothyroxine, the recommended principle is:
- Start high and taper rapidly
- Start low and titrate slowly
- Abrupt withdrawal is preferred to test sensitivity
- Give only liothyronine to minimize cardiac risk
Correct Answer: Start low and titrate slowly
Q11. A serious cardiovascular adverse effect associated with overtreatment with thyroid hormone is:
- Atrial fibrillation
- Bradycardia
- Peripheral edema due to sodium retention
- Valvular stenosis
Correct Answer: Atrial fibrillation
Q12. Which of the following is a contraindication or caution for initiating full replacement doses of thyroid hormone?
- Recent myocardial infarction
- Mild seasonal allergies
- Controlled hypertension
- Vitamin D deficiency
Correct Answer: Recent myocardial infarction
Q13. In myxedema coma, the recommended immediate therapy includes:
- Oral levothyroxine only
- Intravenous levothyroxine +/- liothyronine and supportive care
- High dose oral iodine alone
- Beta blockers and fluid restriction only
Correct Answer: Intravenous levothyroxine +/- liothyronine and supportive care
Q14. Which hepatic enzyme inducer is likely to increase thyroid hormone clearance and require higher replacement doses?
- Rifampin
- Fluoxetine
- Spironolactone
- Allopurinol
Correct Answer: Rifampin
Q15. The majority of circulating T4 is bound to:
- Albumin exclusively
- Thyroxine-binding globulin (TBG)
- Low-density lipoprotein
- Unbound as free hormone
Correct Answer: Thyroxine-binding globulin (TBG)
Q16. Which food interaction is important when counseling patients on levothyroxine administration?
- Take levothyroxine with a high-fiber meal to improve absorption
- Take levothyroxine on an empty stomach, 30–60 minutes before breakfast
- Take levothyroxine with coffee to increase bioavailability
- Avoid vitamin C because it increases levothyroxine clearance
Correct Answer: Take levothyroxine on an empty stomach, 30–60 minutes before breakfast
Q17. Which lab pattern suggests overreplacement with levothyroxine?
- High TSH, low free T4
- Low TSH, high free T4
- Normal TSH with low free T4
- High TSH with high reverse T3
Correct Answer: Low TSH, high free T4
Q18. Which pregnancy-related statement about levothyroxine is true?
- Thyroid hormone requirements usually decrease during pregnancy
- Levothyroxine is contraindicated in pregnancy
- Thyroid hormone requirements often increase during pregnancy
- Liothyronine is preferred over levothyroxine in pregnancy
Correct Answer: Thyroid hormone requirements often increase during pregnancy
Q19. Co-administration of levothyroxine may potentiate the effect of which anticoagulant?
- Heparin
- Warfarin
- Clopidogrel
- Low molecular weight dextran
Correct Answer: Warfarin
Q20. Which adverse effect is commonly associated with chronic excessive thyroid hormone replacement in postmenopausal women?
- Osteoporosis
- Hypertrophic gastritis
- Renal tubular acidosis
- Hirsutism
Correct Answer: Osteoporosis
Q21. In patients with central (secondary) hypothyroidism due to pituitary disease, monitoring therapy should focus on:
- Serum TSH levels only
- Clinical response and free T4 rather than TSH
- Urinary iodine excretion
- Serum total T3 exclusively
Correct Answer: Clinical response and free T4 rather than TSH
Q22. The effect of cholestyramine on levothyroxine is to:
- Increase its renal excretion
- Reduce oral absorption by binding it in the gut
- Enhance conversion of T4 to T3
- Stabilize plasma T4 and prolong half-life
Correct Answer: Reduce oral absorption by binding it in the gut
Q23. Which symptom is more indicative of hypothyroidism than hyperthyroidism?
- Weight gain and cold intolerance
- Tachycardia and heat intolerance
- Fine tremor and anxiety
- Increased sweating and diarrhea
Correct Answer: Weight gain and cold intolerance
Q24. Which laboratory change is typical with adequate levothyroxine replacement in primary hypothyroidism?
- Persistently elevated TSH despite normal free T4
- Suppressed TSH below reference with low free T4
- Normalized TSH with normal free T4
- Markedly elevated reverse T3 with normal TSH
Correct Answer: Normalized TSH with normal free T4
Q25. Which of the following medications can decrease serum thyroid hormone levels by increasing hepatic metabolism?
- Carbamazepine
- Metformin
- Losartan
- Levocetirizine
Correct Answer: Carbamazepine
Q26. The pharmacological rationale for using liothyronine in selected cases is:
- Its longer duration reduces dosing frequency
- Rapid onset is useful when rapid metabolic effect is needed
- It is safer in cardiac disease
- It prevents hypoglycemia in diabetic patients
Correct Answer: Rapid onset is useful when rapid metabolic effect is needed
Q27. A patient taking levothyroxine reports starting a new iron supplement. What counseling is appropriate?
- Take iron and levothyroxine together with breakfast for best effect
- Separate iron and levothyroxine by at least 4 hours to avoid reduced absorption
- Stop levothyroxine while using iron supplements
- Double the levothyroxine dose during iron therapy
Correct Answer: Separate iron and levothyroxine by at least 4 hours to avoid reduced absorption
Q28. Which of the following is NOT an approved therapeutic indication for levothyroxine?
- Treatment of primary hypothyroidism
- Suppression of TSH in thyroid cancer management
- Weight loss in euthyroid obese patients
- Treatment of myxedema coma (parenteral form)
Correct Answer: Weight loss in euthyroid obese patients
Q29. After initiating or changing levothyroxine dose in an adult, when is it appropriate to check TSH to assess the new dose?
- Within 24 hours
- 1–2 weeks
- 4–6 weeks
- After one year only
Correct Answer: 4–6 weeks
Q30. Which statement about levothyroxine protein binding is correct?
- Most circulating levothyroxine is unbound and free
- Levothyroxine is highly protein-bound, primarily to TBG
- Protein binding is negligible and clinically irrelevant
- Levothyroxine binds primarily to albumin only
Correct Answer: Levothyroxine is highly protein-bound, primarily to TBG
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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