Question
A 32-year-old woman attends clinic with weight loss, sweating, fine tremor and episodes of rapid heartbeat. Blood tests confirm thyrotoxicosis, and she is found to have autoimmune hyperthyroidism. She is started on carbimazole.
What is the main mechanism by which carbimazole works?
A. Inhibits peripheral conversion of T4 to T3 by blocking 5′-deiodinase
B. Promotes urinary elimination of thyroid hormones
C. Prevents iodide entry into the thyroid follicular cell
D. Blocks thyroid peroxidase-mediated iodination and coupling reactions on thyroglobulin
E. Stimulates enzymatic degradation of thyroxine within the thyroid gland
Answer
D. Blocks thyroid peroxidase-mediated iodination and coupling reactions on thyroglobulin
Detailed explanation
Carbimazole is an antithyroid drug used in the treatment of thyrotoxicosis, especially Graves’ disease.
It is converted in the body to its active metabolite, methimazole. Its main action is inhibition of thyroid peroxidase.
Thyroid peroxidase is essential for two key steps in thyroid hormone synthesis:
- Iodination of tyrosine residues on thyroglobulin
- Coupling of iodinated tyrosines to form T3 and T4
By blocking thyroid peroxidase, carbimazole reduces new thyroid hormone synthesis.
Important point: carbimazole does not destroy thyroid hormone already stored in the thyroid gland. This is why clinical improvement takes days to weeks, because the thyroid gland already contains preformed T3 and T4.
Why the other options are wrong
A. Inhibits peripheral conversion of T4 to T3 by blocking 5′-deiodinase
This is mainly associated with propylthiouracil, not carbimazole. Propylthiouracil inhibits thyroid peroxidase and also reduces peripheral conversion of T4 to T3 by inhibiting 5′-deiodinase.
B. Promotes urinary elimination of thyroid hormones
This is not the mechanism of carbimazole. Carbimazole reduces synthesis of new thyroid hormone; it does not treat thyrotoxicosis by increasing renal excretion of thyroid hormones.
C. Prevents iodide entry into the thyroid follicular cell
Iodide uptake into thyroid cells occurs through the sodium-iodide symporter. Carbimazole does not primarily block iodide uptake. It blocks the thyroid peroxidase step after iodide has entered the thyroid gland.
D. Blocks thyroid peroxidase-mediated iodination and coupling reactions on thyroglobulin
This is correct. Carbimazole blocks thyroid peroxidase, thereby preventing organification of iodine and coupling of iodotyrosines.
E. Stimulates enzymatic degradation of thyroxine within the thyroid gland
Thyroid peroxidase is involved in thyroid hormone synthesis, not thyroxine breakdown. Carbimazole does not increase degradation of T4.
Key exam point
Carbimazole inhibits thyroid peroxidase, preventing iodination of tyrosine residues on thyroglobulin and coupling of iodotyrosines, thereby reducing T3 and T4 synthesis.
Very important concept / treatment point / clinical trap
The classic dangerous adverse effect of carbimazole is agranulocytosis. Patients must be warned to stop the drug and seek urgent medical advice if they develop fever, sore throat, mouth ulcers or signs of infection.
Cheat sheet for exam
- Carbimazole is used for thyrotoxicosis, especially Graves’ disease.
- Active metabolite: methimazole.
- Mechanism: inhibits thyroid peroxidase.
- Blocks iodination of tyrosine residues on thyroglobulin.
- Blocks coupling of iodotyrosines to form T3 and T4.
- Does not block release of stored thyroid hormone.
- Clinical effect is delayed because preformed hormone remains stored.
- Propylthiouracil also inhibits thyroid peroxidase.
- Propylthiouracil additionally inhibits peripheral T4 to T3 conversion.
- Serious adverse effect: agranulocytosis.
- Other adverse effects: rash, pruritus, arthralgia, GI upset, abnormal LFTs.
- Carbimazole crosses the placenta.
- Propylthiouracil is often preferred in the first trimester of pregnancy.
- Carbimazole may be used later in pregnancy at the lowest effective dose.
- Warn patients about sore throat, fever and mouth ulcers.
Flash cards
Q: What enzyme does carbimazole inhibit?
A: Thyroid peroxidase. This reduces thyroid hormone synthesis.
Q: Which two thyroid synthesis steps are blocked by carbimazole?
A: Iodination of tyrosine residues and coupling of iodotyrosines on thyroglobulin.
Q: Why does carbimazole take time to improve symptoms?
A: It blocks new hormone synthesis but does not remove already stored T3 and T4.
Q: Which antithyroid drug also inhibits peripheral conversion of T4 to T3?
A: Propylthiouracil.
Q: What is the most important serious adverse effect of carbimazole?
A: Agranulocytosis.
Q: What symptoms should a patient on carbimazole report urgently?
A: Fever, sore throat, mouth ulcers or other signs of infection.
Q: What is the active metabolite of carbimazole?
A: Methimazole.
Q: Does carbimazole block iodide uptake into the thyroid?
A: No. It blocks thyroid peroxidase-mediated organification and coupling.
MCQs
- A patient with Graves’ disease is started on carbimazole. Which enzyme is directly inhibited by this drug?
A. Thyroglobulin protease
B. Thyroid peroxidase
C. 5′-deiodinase
D. Sodium-iodide symporter
E. TSH receptor kinase
Answer: B. Thyroid peroxidase
Explanation: Carbimazole inhibits thyroid peroxidase, preventing iodination and coupling reactions required for T3 and T4 synthesis.
- Which of the following best explains why carbimazole does not produce immediate biochemical euthyroidism?
A. It only acts after renal activation
B. It initially increases TSH secretion
C. It does not affect preformed thyroid hormone stores
D. It has poor oral absorption
E. It requires iodine repletion before working
Answer: C. It does not affect preformed thyroid hormone stores
Explanation: Carbimazole prevents new hormone synthesis but does not remove hormone already stored in the thyroid gland. Therefore, symptomatic and biochemical improvement is delayed.
- Which antithyroid drug has an additional action of reducing peripheral conversion of T4 to T3?
A. Carbimazole
B. Methimazole
C. Levothyroxine
D. Propylthiouracil
E. Radioiodine
Answer: D. Propylthiouracil
Explanation: Propylthiouracil inhibits thyroid peroxidase and also inhibits 5′-deiodinase, reducing peripheral conversion of T4 to T3. This makes it useful in some severe thyrotoxic states.
- Which of the following is false regarding carbimazole?
A. It is used in Graves’ disease
B. It inhibits thyroid peroxidase
C. It blocks coupling of iodotyrosines
D. It may cause agranulocytosis
E. It directly blocks TSH receptor antibodies
Answer: E. It directly blocks TSH receptor antibodies
Explanation: Carbimazole reduces thyroid hormone synthesis but does not directly block TSH receptor antibodies. Graves’ disease is caused by stimulating TSH receptor antibodies, but carbimazole acts downstream at hormone synthesis.
- A woman taking carbimazole develops fever and a painful sore throat. What is the most important concern?
A. Thyroid storm
B. Agranulocytosis
C. Iodine deficiency
D. Hypercalcaemia
E. Addisonian crisis
Answer: B. Agranulocytosis
Explanation: Fever and sore throat in a patient taking carbimazole should raise urgent concern for agranulocytosis. The drug should be stopped and urgent full blood count arranged.
- Which reaction is mainly prevented by inhibition of thyroid peroxidase?
A. Conversion of T3 to reverse T3
B. Release of TSH from the pituitary
C. Iodination of tyrosine residues on thyroglobulin
D. Renal clearance of thyroxine
E. Binding of T4 to albumin
Answer: C. Iodination of tyrosine residues on thyroglobulin
Explanation: Thyroid peroxidase catalyses iodination of tyrosine residues and coupling of iodotyrosines. Carbimazole blocks this enzyme.
- Which of the following is false about propylthiouracil compared with carbimazole?
A. It inhibits thyroid peroxidase
B. It can reduce peripheral T4 to T3 conversion
C. It is often preferred in the first trimester of pregnancy
D. It is completely free of hepatotoxicity risk
E. It is used in thyrotoxicosis
Answer: D. It is completely free of hepatotoxicity risk
Explanation: Propylthiouracil can cause hepatotoxicity, rarely severe. It is not risk-free. It does inhibit thyroid peroxidase and 5′-deiodinase.
- Carbimazole mainly reduces levels of thyroid hormone by interfering with which structure?
A. Thyroglobulin
B. Parathyroid chief cells
C. Adrenal cortex
D. Posterior pituitary
E. Hypothalamic TRH neurons
Answer: A. Thyroglobulin
Explanation: Carbimazole blocks thyroid peroxidase-mediated iodination and coupling of tyrosine residues on thyroglobulin. Thyroglobulin is the scaffold on which thyroid hormone synthesis occurs.
- A patient on carbimazole asks why she was also prescribed propranolol initially. What is the best explanation?
A. Propranolol cures Graves’ disease
B. Propranolol blocks thyroid peroxidase
C. Propranolol controls adrenergic symptoms while carbimazole takes effect
D. Propranolol increases carbimazole absorption
E. Propranolol prevents agranulocytosis
Answer: C. Propranolol controls adrenergic symptoms while carbimazole takes effect
Explanation: Beta-blockers such as propranolol improve tremor, palpitations and anxiety while antithyroid drugs gradually reduce hormone synthesis.
- Which of the following is false regarding thyroid hormone synthesis?
A. Iodide is taken into thyroid follicular cells
B. Thyroid peroxidase helps oxidise iodide
C. Iodination occurs on tyrosine residues of thyroglobulin
D. Coupling of iodotyrosines forms T3 and T4
E. Carbimazole increases coupling of iodotyrosines
Answer: E. Carbimazole increases coupling of iodotyrosines
Explanation: Carbimazole decreases coupling by inhibiting thyroid peroxidase. It therefore reduces T3 and T4 synthesis.
Summary for quick exam revision
Carbimazole is a key antithyroid drug used in thyrotoxicosis, especially Graves’ disease. It is converted to methimazole, its active metabolite. Its main mechanism is inhibition of thyroid peroxidase. Thyroid peroxidase is required for iodination of tyrosine residues on thyroglobulin and coupling of iodotyrosines to form T3 and T4. Carbimazole therefore reduces new thyroid hormone synthesis. It does not block the release of already stored thyroid hormone, so improvement is not immediate. Propylthiouracil also inhibits thyroid peroxidase, but unlike carbimazole, it also inhibits peripheral conversion of T4 to T3 by blocking 5′-deiodinase. This difference is a common exam trap. Carbimazole does not block iodide uptake into the thyroid follicular cell. It also does not increase renal excretion or breakdown of thyroid hormone. The most important dangerous adverse effect is agranulocytosis. Patients should be warned to stop the drug and seek urgent medical help if they develop fever, sore throat, mouth ulcers or signs of infection. Carbimazole crosses the placenta, so pregnancy requires careful drug choice and dose adjustment. In exams, the phrase “blocks thyroid peroxidase” should immediately make you think of reduced iodination and coupling on thyroglobulin.