Usage
- Medical Conditions: Carbimazole is prescribed for hyperthyroidism (overactive thyroid). It is used to reduce the production of thyroid hormones. It is also used in preparation for thyroidectomy or radioactive iodine treatment in hyperthyroidism.
- Pharmacological Classification: Antithyroid agent.
- Mechanism of Action: Carbimazole inhibits the synthesis of thyroid hormones (T3 and T4) by preventing iodine from binding to tyrosine residues on thyroglobulin, a precursor to thyroid hormones. It does not affect the release of preformed thyroid hormones.
Alternate Names
- International/Regional Variations: Methimazole (active metabolite of carbimazole).
- Brand Names: Neo-Mercazole.
How It Works
- Pharmacodynamics: Carbimazole reduces the production of thyroid hormones, thus normalizing metabolic rate, heart rate, and other symptoms of hyperthyroidism.
- Pharmacokinetics: Carbimazole is rapidly absorbed orally and is almost completely metabolized to its active form, methimazole. Methimazole is well-distributed throughout the body and crosses the placenta and is found in breastmilk. It is primarily excreted by the kidneys, with a small amount metabolized by the liver.
- Mode of Action: Carbimazole’s active metabolite, methimazole, inhibits the enzyme thyroid peroxidase, which is essential for the iodination and coupling of tyrosine residues on thyroglobulin, thus reducing the synthesis of thyroid hormones.
- Receptor Binding/Enzyme Inhibition: Inhibits thyroid peroxidase enzyme.
- Elimination Pathways: Renal excretion (primary) and some hepatic metabolism.
Dosage
Standard Dosage
Adults:
- Initial dose: 20-60 mg daily, divided into 2-3 doses.
- Maintenance dose: 5-15 mg daily, often as a single dose. The dose is titrated based on thyroid function tests.
Children (3-17 years):
- Initial dose: 15mg daily, adjusted based on response and thyroid function tests.
- Children under 2 years: Not recommended due to limited data on safety and efficacy in this age group.
Special Cases:
- Elderly Patients: No specific dosage adjustment is required, but monitor closely due to increased risk of adverse events like neutropenia.
- Patients with Renal Impairment: No specific dose adjustments are typically recommended, but close monitoring is necessary.
- Patients with Hepatic Dysfunction: Contraindicated in severe hepatic impairment. Use with caution in mild to moderate impairment.
- Patients with Comorbid Conditions: Use with caution in patients with pre-existing hematological conditions.
Clinical Use Cases Carbimazole’s use is primarily limited to the management of hyperthyroidism. It is not typically used in the clinical scenarios mentioned below:
- Intubation: Not applicable.
- Surgical Procedures: Used pre-operatively to achieve euthyroidism before thyroidectomy for hyperthyroidism.
- Mechanical Ventilation: Not applicable.
- Intensive Care Unit (ICU) Use: Not applicable.
- Emergency Situations: Not applicable.
Dosage Adjustments
Dosage adjustments are made based on thyroid function test results (TSH, free T4) and individual patient response and tolerance.
Side Effects
Common Side Effects:
- Nausea, vomiting, diarrhea
- Headache
- Dizziness
- Itchy skin, rash
- Joint pain
- Hair thinning
Rare but Serious Side Effects:
- Agranulocytosis (severely low white blood cell count) manifested by sore throat, fever, mouth ulcers
- Acute pancreatitis (severe abdominal pain)
- Jaundice (yellowing of skin and whites of eyes)
- Hepatotoxicity (liver damage)
- Bone marrow depression
Long-Term Effects: While generally safe for long-term use, regular monitoring of thyroid function and complete blood count is required.
Adverse Drug Reactions (ADR): Agranulocytosis, hepatotoxicity, acute pancreatitis.
Contraindications
- Hypersensitivity to carbimazole or other thionamides (propylthiouracil, methimazole)
- Severe liver disease
- History of carbimazole/methimazole induced agranulocytosis or pancreatitis
- Breastfeeding (unless absolutely necessary and under close monitoring)
Drug Interactions
- Anticoagulants (e.g., warfarin): Carbimazole can increase the effect of anticoagulants, leading to increased bleeding risk.
- Theophylline: Carbimazole can increase theophylline levels.
- Digoxin: Carbimazole may increase digoxin levels.
- Beta-blockers: Often co-administered with carbimazole to manage the symptoms of hyperthyroidism.
- Steroids (e.g., prednisolone): Monitor for potential interactions.
- Erythromycin: May affect carbimazole metabolism.
Pregnancy and Breastfeeding
- Pregnancy Safety Category: Carbimazole can cause birth defects. Propylthiouracil is generally preferred during pregnancy, especially in the first trimester. However, the lowest effective dose of carbimazole might be used if PTU is not tolerated. Close monitoring of thyroid function in both mother and fetus is crucial.
- Breastfeeding: Carbimazole passes into breast milk, but in small amounts. Limited data suggests that at doses up to 30mg daily, it is unlikely to affect the infant’s thyroid function. Monitor infant’s thyroid function if higher doses are necessary.
Drug Profile Summary
- Mechanism of Action: Inhibits thyroid hormone synthesis by blocking thyroid peroxidase.
- Side Effects: Nausea, rash, joint pain, rarely agranulocytosis.
- Contraindications: Severe liver disease, hypersensitivity, history of drug-induced agranulocytosis.
- Drug Interactions: Anticoagulants, theophylline, digoxin.
- Pregnancy & Breastfeeding: Avoid during pregnancy unless absolutely necessary; use caution during breastfeeding. Monitor infant’s thyroid levels.
- Dosage: Adults: Initial 20-60mg/day, maintenance 5-15mg/day. Children: Initial 15mg/day, adjusted based on thyroid function.
- Monitoring Parameters: Thyroid function tests (TSH, free T4, T3), complete blood count (especially during initial treatment).
Popular Combinations
- Beta-blockers (e.g., propranolol) to manage the symptoms of hyperthyroidism until carbimazole takes effect.
- Levothyroxine (in “block and replace” regimen) to ensure euthyroidism while blocking endogenous thyroid hormone production with higher doses of carbimazole.
Precautions
- General Precautions: Monitor for signs of agranulocytosis (sore throat, fever), liver dysfunction, and other adverse effects. Regular blood tests (thyroid function tests and complete blood count) are essential.
- Specific Populations: Close monitoring during pregnancy (if absolutely necessary) and breastfeeding. Careful monitoring in the elderly and those with pre-existing hematological conditions.
- Lifestyle Considerations: None specifically noted.
FAQs (Frequently Asked Questions)
Q1: What is the recommended dosage for Carbimazole?
A: Adults: 20-60 mg/day initially, then reduced to 5-15 mg/day for maintenance. Children (3-17 years): 15 mg/day initially, adjusted based on response.
Q2: What are the most serious side effects of Carbimazole?
A: Agranulocytosis (low white blood cell count), liver damage, and acute pancreatitis.
Q3: Can Carbimazole be used during pregnancy?
A: It should generally be avoided, especially in the first trimester, due to a risk of birth defects. Propylthiouracil is usually preferred. If Carbimazole is used, it should be the lowest effective dose and under close monitoring.
Q4: Can Carbimazole be used during breastfeeding?
A: Carbimazole passes into breast milk. If the maternal dose is less than 30mg daily, the risk to the infant is minimal. Infant thyroid monitoring is advisable, especially if using higher doses.
Q5: How does Carbimazole work?
A: It inhibits the enzyme thyroid peroxidase, blocking the synthesis of thyroid hormones (T3 and T4).
Q6: How long does it take for Carbimazole to work?
A: Patients usually start to feel better within a few weeks, but it can take several months to achieve normal thyroid hormone levels.
Q7: What should patients do if they experience a sore throat or fever while taking Carbimazole?
A: They should stop the medication immediately and contact their doctor, as this may indicate agranulocytosis.
Q8: What are the common drug interactions with Carbimazole?
A: Anticoagulants (e.g., warfarin), theophylline, and digoxin.
Q9: What should be monitored in patients taking Carbimazole?
A: Thyroid function tests (TSH, Free T4, T3), Complete Blood Count.
Q10: Is Carbimazole safe for long-term use?
A: Yes, but regular monitoring of thyroid function and blood counts is necessary.
This information is current as of February 16, 2025. As medical knowledge is constantly evolving, it is essential to consult updated resources and guidelines for the latest information.
