Dr Sharon Rees @reesprescribe
Day 1: Sulphur-containing compounds were found to be thyroid-toxic in the 1940s, which led to propylthiouracil use. A second thiouracil analogue ‘methimazole’ was found and its prodrug #carbimazole was licensed in 1953, as it was believed to be less toxic, which was not the case.
Dr Sharon Rees @reesprescribe
Day 2: #carbimazole is used to treat hyperthyroidism for all ages. The adult starting dose is 20–60 mg daily (in divided doses) and the child dose is dependent on age/weight; all regimens are based on time to euthyroid, then reduce to the maintenance dose. #carbimazole is also used for the ‘blocking-replacement’ regimen in combination for 6–18 months with levothyroxine to prevent hypothyroidism.
Dr Sharon Rees @reesprescribe
Day 3: Oral absorption is unaffected by food; metabolite cmax 1–2 hours. #carbimazole has a low volume of distribution as it concentrates in the thyroid gland. Crosses the placenta and presents in breast milk. 10% entero-hepatic recycling and contraindicated if severe hepatic impairment. T½ 5–6 hrs (prolonged in hyperthyroidism). Renal excretion is approximately 90% as the drug is mostly unchanged.
Dr Sharon Rees @reesprescribe
Day 4: The full mechanism of action for #carbimazole is not fully understood; there is reduced uptake of iodine in the thyroid gland and it also opposes iodination by inhibition of thyroid peroxidases; these actions lead to thyroid suppression and reduced thyroxine synthesis. Symptoms can take a few weeks to abate.
Dr Sharon Rees @reesprescribe
Day 5: Poor frequency data for adverse drug events. Common adverse drug events include nausea, headache and dizziness. Rare adverse drug events include agranulocytosis. Bone marrow suppression and blood disorders can be fatal; pre-existing significant haematological conditions preclude use. Advise re signs of bone marrow suppression requiring urgent attention, such as fever and bruising. It should be noted that #carbimazole can worsen goitre (NOT exhaustive).
Dr Sharon Rees @reesprescribe
Day 6: #carbimazole was licensed pre-vigilance data. No drug–drug interaction is denoted as ‘severe’ in the BNF. As a vitamin K antagonist, #carbimazole could increase anti-coagulation (monitor); slowing kinetic processes could alter drug levels, leading to increased lithium, digoxin and prednisolone levels (NOT exhaustive).
Dr Sharon Rees @reesprescribe
Day 7: Propylthiouracil is a first-line medication in the US, because it is less easily transferred across the placenta. #carbimazole not first line in the UK for pregnancy as there is the potential for fetal harm, especially during the first trimester. Effective contraception is advised to avoid unintentional pregnancy.
Dr Sharon Rees @reesprescribe
Day 7 (contd): The Summary of Product Characteristics was altered in 2018 following EU safety recommendations, but acknowledges evidence is ‘based on human experience from epidemiological studies and spontaneous reporting’.
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In addition to the tweets, read the BNF treatment summary on hyperthyroidism and the monograph for carbimazole, as well as the Summary of Product Characteristics for carbimazole, and then answer the 10 questions. Please submit the answers to reesprescribe@gmail.com as a numbered list with TRUE/FALSE or the correct A,B,C,D option. If you achieve 8 or more out of 10 on the questions, a CPD certificate will be emailed to you.
Further reading on #carbimazole
- Carbimazole 20 mg tablets: https://www.medicines.org.uk/emc/product/4266/smpc
- Hyperthyroidism: https://bnf.nice.org.uk/treatment-summaries/hyperthyroidism/
- Carbimazole 20 mg tablets: https://www.medicines.org.uk/emc/product/4266/smpc
- Carbimazole was discovered as an anti-thyroid function drug in the 1970sTRUE or FALSE?
- Carbimazole is a prodrugTRUE or FALSE?
- Carbimazole is not first line in pregnancyTRUE or FALSE?
- Which of the following is TRUE?
- Carbimazole concentrates in the pituitary gland
- Carbimazole is broken down extensively in the liver
- Carbimazole has very high protein binding characteristics
- Carbimazole crosses the placenta
- Which of the following is a rare adverse drug effect
- Aplastic anaemia
- Agranulocytosis
- Rash
- Loss of taste
- Which of the following best describes the mechanism of action for carbimazole?
- It inhibits thyroid peroxidase function
- It is directly cytotoxic to thyroid producing cells
- It limits the absorption of dietary iodine
- It causes immune suppression stopping any attack on the thyroid gland
- Carbimazole usually takes months to years to improve symptomsTRUE or FALSE?
- Carbimazole has no listed ‘severe’ drug–drug interactions listed in the BNFTRUE or FALSE?
- Counselling about the relevance of signs/symptoms such as severe sore throat, fever, bruising etc is important because these can indicate bone marrow suppressionTRUE or FALSE?
- Carbimazole can cause congenital malformations if used in pregnancyTRUE or FALSE?