Thyroid Drugs and Antithyroid Agents — MCQs
Regarding thyroid hormone all are true except:
Which of the antithyroid drugs inhibits iodine trapping?
Which of the following conditions is the most common complication of radioiodine treatment for Graves' disease?
A female presents with a 1 × 1 cm thyroid swelling. What is the next best step in management?
Treatment for childhood hypothyroidism is with -
Which antithyroid drug is safer during the first trimester of pregnancy?
A 47-year-old woman presents with complaints of nervousness and increased sensitivity to hot weather. She is diagnosed with hyperthyroidism and prescribed propylthiouracil. What is the principal mechanism by which this drug acts?
Which of the following statements about thyroid hormone receptors is correct?
Which of the following drugs does not act on the thyroid?
Preoperative medication of thyrotoxicosis are all except?
Thyroid Drugs and Antithyroid Agents Indian Medical PG Practice Questions and MCQs
Question 1: Regarding thyroid hormone all are true except:
- A. T4 has the maximum plasma concentration
- B. T3 is more avidly bound to nuclear receptors than T4
- C. T3 is more active than T4
- D. T4 has shorter half life than T3 (Correct Answer)
Explanation: ***T4 has shorter half-life than T3*** - This statement is incorrect because **T4 (thyroxine) has a significantly longer half-life (approximately 7 days)** compared to **T3 (triiodothyronine), which has a half-life of about 1-2 days**. - The longer half-life of T4 allows for a more stable and sustained effect, acting as a prohormone. *T4 has the maximum plasma concentration* - **T4 is secreted in much larger quantities from the thyroid gland than T3 (about 80% T4 vs. 20% T3)**, leading to a higher concentration in the plasma. - This high plasma concentration of T4 makes it the primary circulating thyroid hormone, mostly bound to plasma proteins. *T3 is more avidly bound to nuclear receptors than T4* - **T3 binds to nuclear thyroid hormone receptors with 10 to 15 times greater affinity than T4**, making it the more potent and biologically active form. - This stronger binding affinity is crucial for T3's direct physiological effects on target cells. *T3 is more active than T4* - **T3 is considered the metabolically active form of thyroid hormone**, directly mediating most physiological effects by binding to nuclear receptors. - T4 acts largely as a **prohormone**, being deiodinated in peripheral tissues to form T3, which then exerts metabolic activity.
Question 2: Which of the antithyroid drugs inhibits iodine trapping?
- A. Radioactive iodine
- B. Iodides
- C. Thiocyanates (Correct Answer)
- D. Methimazole
Explanation: ***Thiocyanates*** - **Thiocyanates** are competitive inhibitors of the **sodium-iodide symporter (NIS)**, which is responsible for actively transporting iodide into thyroid follicular cells (iodine trapping) [1]. - By blocking NIS, thiocyanates prevent the thyroid gland from accumulating iodide, thereby inhibiting thyroid hormone synthesis [1]. *Radioactive iodine* - **Radioactive iodine (RAI)** primarily works by being taken up by thyroid cells and emitting **beta particles**, which destroy the thyroid tissue [3]. - It does not inhibit iodine trapping, but rather uses the trapping mechanism to concentrate in the thyroid and exert its destructive effect [3]. *Iodides* - **Iodides** (e.g., Lugol's solution) paradoxically inhibit organification and hormone release from the thyroid gland, an effect known as the **Wolff-Chaikoff effect** [2]. - They also decrease the vascularity and size of the thyroid gland, making them useful in preparing patients for thyroidectomy, but do not directly inhibit iodine trapping [2]. *Methimazole* - **Methimazole** is a **thionamide** drug that primarily inhibits the enzyme **thyroid peroxidase**. - This prevents the **organification of iodide** (incorporation of iodine into tyrosine residues) and the **coupling of iodotyrosines** (forming T3 and T4), not the initial trapping of iodine.
Question 3: Which of the following conditions is the most common complication of radioiodine treatment for Graves' disease?
- A. Hypothyroidism (Correct Answer)
- B. Thyroid cancer
- C. Thyroid storm
- D. Subacute thyroiditis
Explanation: ***Hypothyroidism*** - **Radioiodine (RAI) therapy** destroys overactive thyroid cells, making it highly effective for Graves' disease but often leading to a permanent state of **hypothyroidism** post-treatment. - The goal of RAI is to eliminate the source of excess hormone production, and while effective, it frequently necessitates lifelong **thyroid hormone replacement**. *Thyroid storm* - **Thyroid storm** is a rare, life-threatening complication, usually seen in untreated or undertreated hyperthyroidism or during acute stress, not typically a direct outcome of effective RAI. - While a transient increase in thyroid hormones can occur shortly after RAI, a full-blown thyroid storm is infrequent with proper preparation and management. *Thyroid cancer* - There is no significant evidence to suggest an increased risk of **thyroid cancer** in adults following therapeutic doses of radioiodine for Graves' disease [1]. - The radiation dose is targeted primarily at the thyroid gland, and studies have shown no clear link to increased malignancy [1]. *Subacute thyroiditis* - **Subacute thyroiditis** (also known as de Quervain's thyroiditis) is typically a post-viral inflammatory condition of the thyroid, characterized by pain and tenderness in the thyroid gland [2]. - It does not directly result from radioiodine treatment; however, some patients may experience a transient inflammatory response (radiation thyroiditis) after RAI, which is usually mild and self-limiting, not true subacute thyroiditis.
Question 4: A female presents with a 1 × 1 cm thyroid swelling. What is the next best step in management?
- A. I-131
- B. TSH (Correct Answer)
- C. TSH & T4
- D. T3 & T4
- E. FNAC
Explanation: ***Correct Option: TSH*** - **Thyroid-stimulating hormone (TSH)** is the most sensitive initial test to assess thyroid function when a thyroid nodule is discovered. - An abnormal TSH level (either high or low) can guide further investigation into whether the nodule is associated with a functional thyroid disorder. - **TSH should be the first test** according to American Thyroid Association guidelines for thyroid nodule evaluation. *Incorrect Option: I-131* - **I-131 (radioactive iodine therapy)** is a treatment modality for hyperthyroidism or thyroid cancer, not a diagnostic step for initial thyroid swelling evaluation. - Administering I-131 before assessing thyroid function would be inappropriate and could lead to unnecessary or harmful intervention. *Incorrect Option: TSH & T4* - While TSH is crucial, adding **T4 (thyroxine)** as an initial step is often not necessary if TSH is normal, as TSH alone effectively screens for primary thyroid dysfunction. - Measuring both TSH and T4 is typically reserved for situations where TSH is abnormal or when central hypothyroidism is suspected. *Incorrect Option: T3 & T4* - Measuring **T3 (triiodothyronine)** along with T4 as an initial screening for a thyroid nodule is generally not recommended. - T3 levels are primarily used to diagnose **hyperthyroidism** or to evaluate the severity of thyrotoxicosis after an abnormal TSH and T4 have been identified. *Incorrect Option: FNAC* - While **Fine Needle Aspiration Cytology (FNAC)** is an essential diagnostic tool for thyroid nodules, it is typically performed after TSH assessment. - FNAC is indicated for nodules >1 cm with suspicious ultrasound features, but **functional assessment with TSH comes first** to rule out hyperfunctioning nodules.
Question 5: Treatment for childhood hypothyroidism is with -
- A. TSH
- B. Propylthiouracil
- C. Levothyroxine (Correct Answer)
- D. T3
Explanation: ***Correct: Levothyroxine*** - **Levothyroxine** is synthetic **thyroxine (T4)**, the standard hormone replacement therapy for childhood hypothyroidism - It is **well-absorbed orally**, has a **long half-life**, and provides stable thyroid hormone levels - Once administered, it is converted to **T3 (the active form)** in peripheral tissues, ensuring steady thyroid hormone supply - **Preferred in children** for consistent growth and neurodevelopment *Incorrect: TSH* - **TSH (Thyroid-Stimulating Hormone)** is produced by the pituitary gland to stimulate thyroid hormone production - In **primary hypothyroidism**, TSH levels are *elevated* due to lack of negative feedback - TSH is a **diagnostic marker**, not a therapeutic agent for hormone replacement *Incorrect: Propylthiouracil* - **Propylthiouracil** is an **anti-thyroid drug** used to treat **hyperthyroidism** (excessive thyroid hormone) - It works by *inhibiting* thyroid hormone synthesis - Using it in hypothyroidism would **worsen** the condition by further reducing thyroid hormone levels *Incorrect: T3* - **T3 (triiodothyronine)** is the more metabolically active form of thyroid hormone - It has a **shorter half-life** and causes more **fluctuating hormone levels** - **Not preferred** for long-term replacement in children due to difficulty maintaining stable levels - Most levothyroxine (T4) is naturally converted to T3 *in vivo*, providing adequate T3 without direct supplementation
Question 6: Which antithyroid drug is safer during the first trimester of pregnancy?
- A. Radioactive iodine
- B. Carbimazole
- C. Propylthiouracil (Correct Answer)
- D. Methimazole
- E. Propranolol
Explanation: ***Propylthiouracil*** - **Propylthiouracil (PTU)** is the preferred drug for treating hyperthyroidism in the **first trimester of pregnancy** due to a lower risk of teratogenic effects compared to methimazole or carbimazole. - While PTU carries a risk of hepatic toxicity, it is generally favored during early pregnancy to avoid the established teratogenic risks of other thionamides. *Radioactive iodine* - **Radioactive iodine (RAI)** is absolutely contraindicated in pregnancy as it crosses the placenta and can cause permanent **fetal hypothyroidism** or **agenesis of the fetal thyroid gland**. - Its use can lead to the destruction of the fetal thyroid, which is unacceptable given the availability of safer alternatives. *Carbimazole* - **Carbimazole** is converted to methimazole in the body and is associated with a higher risk of **fetal embryopathy** during the first trimester, including aplasia cutis, choanal atresia, and esophageal atresia. - It should generally be avoided in the first trimester if possible, switching to PTU instead. *Methimazole* - **Methimazole** is structurally similar to carbimazole and also carries a significant risk of **teratogenic effects** such as **aplasia cutis**, omphalocele, and choanal atresia when used in the first trimester. - It is often reserved for the second and third trimesters if a thionamide is required, or for patients who cannot tolerate PTU, but ideally avoided in early pregnancy. *Propranolol* - **Propranolol** is a beta-blocker used for symptomatic management of hyperthyroidism (controlling tachycardia, tremor, anxiety) but is not an antithyroid drug and does not treat the underlying hyperthyroidism. - While generally safe in pregnancy, it only provides adjunctive therapy and cannot replace definitive antithyroid treatment.
Question 7: A 47-year-old woman presents with complaints of nervousness and increased sensitivity to hot weather. She is diagnosed with hyperthyroidism and prescribed propylthiouracil. What is the principal mechanism by which this drug acts?
- A. Reducing the proteolysis of thyroglobulin.
- B. Inhibiting the binding of TSH to its receptor.
- C. Inhibiting the enzyme thyroid peroxidase, which reduces the synthesis of thyroid hormones. (Correct Answer)
- D. Altering the levels of reverse T3 (rT3) in the body.
Explanation: ***Inhibiting the enzyme thyroid peroxidase, which reduces the synthesis of thyroid hormones.*** - **Propylthiouracil (PTU)** is a **thionamide** drug that primarily acts by inhibiting the enzyme **thyroid peroxidase**. - Thyroid peroxidase is crucial for the **organification of iodide** and the **coupling of iodotyrosines** (MIT and DIT) to form T3 and T4, thus reducing the synthesis of thyroid hormones. *Inhibiting the binding of TSH to its receptor.* - This mechanism is characteristic of **TSH receptor antibodies**, which are a cause of hyperthyroidism (e.g., in Graves' disease), rather than the action of an antithyroid drug like PTU. - PTU works at the level of hormone synthesis within the thyroid gland, not at the receptor level for TSH. *Reducing the proteolysis of thyroglobulin.* - While thyroid hormones are stored as part of thyroglobulin, and their release involves proteolysis, this is not the **principal mechanism of action** for PTU. - The main effect of PTU is upstream, preventing the formation of the hormones themselves. *Altering the levels of reverse T3 (rT3) in the body.* - PTU does inhibit the **peripheral conversion of T4 to T3**, which can reduce overall T3 levels and increase rT3, but this is a **secondary mechanism**. - The primary and most significant action for reducing hyperthyroid symptoms is the direct inhibition of thyroid hormone synthesis within the gland.
Question 8: Which of the following statements about thyroid hormone receptors is correct?
- A. They directly bind to thyrotropin-releasing hormone (TRH)
- B. They directly bind to thyroid-stimulating hormone (TSH)
- C. They cause nuclear transcription after binding with T4
- D. They are intracellular receptors that mediate gene transcription after binding with T3 or T4, but their primary action is through T3. (Correct Answer)
Explanation: ***They are intracellular receptors that mediate gene transcription after binding with T3 or T4, but their primary action is through T3.*** - **Thyroid hormone receptors** are indeed **intracellular** and act as **ligand-activated transcription factors**, regulating gene expression. - While both **T3** and **T4** can bind, **T3 (triiodothyronine)** is the more potent and active form, binding with much higher affinity to the receptors to exert its primary metabolic effects. *They directly bind to thyrotropin-releasing hormone (TRH)* - **TRH (thyrotropin-releasing hormone)** is produced by the hypothalamus and acts on the **pituitary gland** to stimulate TSH release, not directly on thyroid hormone receptors. - Thyroid hormone receptors bind to thyroid hormones (**T3 and T4**), not to the hypothalamic releasing hormones like TRH. *They directly bind to thyroid-stimulating hormone (TSH)* - **TSH (thyroid-stimulating hormone)** is produced by the pituitary gland and primarily acts on receptors located on the **thyroid gland cells** to stimulate thyroid hormone synthesis and release. - Thyroid hormone receptors are distinct from TSH receptors and bind to the hormones themselves (**T3/T4**), not the stimulating hormone TSH. *Causes nuclear transcription after binding with T4* - While **T4 (thyroxine)** can bind to thyroid hormone receptors, it is primarily a **prohormone**. - T4 is largely converted to the more active **T3** within target cells, and **T3** is the main mediator of nuclear transcription through these receptors.
Question 9: Which of the following drugs does not act on the thyroid?
- A. Propranolol (Correct Answer)
- B. Propylthiouracil
- C. Sodium iodide
- D. Thiocyanate
Explanation: ***Propranolol*** - **Propranolol** is a **beta-blocker** primarily used to relieve symptoms of hyperthyroidism such as palpitations, tremor, and anxiety by blocking beta-adrenergic receptors. - It does not directly affect thyroid hormone production or release, making it a symptomatic treatment rather than an antithyroid drug. *Propylthiouracil* - **Propylthiouracil (PTU)** is a **thionamide** drug that directly inhibits the synthesis of thyroid hormones by interfering with the **thyroid peroxidase enzyme**. - It also blocks the peripheral conversion of **thyroxine (T4)** to the more active **triiodothyronine (T3)**. *Sodium iodide* - High doses of **iodine**, such as **sodium iodide**, acutely inhibit thyroid hormone synthesis and release, a phenomenon known as the **Wolff-Chaikoff effect**. - It is often used in preparation for surgery in hyperthyroid patients or during thyroid storm. *Thiocyanate* - **Thiocyanate** is an **anion** that competitively inhibits the **iodide transporter (NIS)** in the thyroid gland. - By blocking the uptake of iodide into thyroid cells, it reduces the availability of iodine for thyroid hormone synthesis.
Question 10: Preoperative medication of thyrotoxicosis are all except?
- A. Carbimazole
- B. PTU
- C. Propranolol
- D. Levothyroxine (Correct Answer)
Explanation: Levothyroxine - Levothyroxine is a synthetic thyroid hormone used to treat hypothyroidism, meaning it increases thyroid hormone levels, which would worsen thyrotoxicosis [1]. - Its administration would be contraindicated in a patient with thyrotoxicosis, as the goal is to reduce thyroid hormone levels preoperatively. Carbimazole - Carbimazole is a thionamide drug that inhibits the synthesis of thyroid hormones, making it a critical medication for treating hyperthyroidism and preparing patients for surgery [1]. - It reduces the amount of thyroid hormone produced by the thyroid gland, thus mitigating the risks associated with thyrotoxicosis during surgery. PTU - Propylthiouracil (PTU), like carbimazole, is a thionamide that blocks thyroid hormone synthesis and also inhibits the conversion of T4 to T3 [1]. - It is used in the preoperative management of thyrotoxicosis to achieve a euthyroid state and prevent a thyroid storm. Propranolol - Propranolol is a beta-blocker used to manage the symptoms of thyrotoxicosis, particularly the cardiovascular effects such as tachycardia, palpitations, and tremors [1]. - While it does not affect thyroid hormone levels directly, it helps control symptoms and stabilize the patient preoperatively, making them a safer candidate for surgery [1].
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