Anti-thyrox 5 (Carbimazole Tablets BP 5 mg)

Indications

• Hyperthyroidism
• Graves' disease
• Thyrotoxicosis

Dosage

Children: For children, carbimazole is used to control hyperthyroidism, generally for 8 to 12 weeks. The specific dose should be determined by a

Adults: 15–40 mg daily, continued until the patient becomes euthyroid, usually after 4 to 8 weeks. Higher doses should be prescribed under specialist supervision only, then reduced to 5–15 mg daily. For blocking-replacement regimen, 40–60 mg daily in combination with levothyroxine is advised, typically for 18 months.

Mechanism of action

Carbimazole decreases the uptake and concentration of inorganic iodine by the thyroid gland. Once metabolized to its active form, methimazole, it inhibits the thyroid peroxidase enzyme, which is crucial for the coupling and iodination of tyrosine residues on thyroglobulin. This inhibition ultimately reduces the synthesis of thyroid hormones T3 (triiodothyronine) and T4 (thyroxine).

Pharmacodynamics

Carbimazole exhibits its antithyroid effects through its conversion to methimazole in the body. This conversion is pivotal in reducing the production of thyroid hormones, thereby managing hyperthyroid states. The drug is effective in controlling symptoms associated with excessive thyroid hormone levels and is often used as a preoperative treatment for patients undergoing thyroidectomy.

Pharmacokinetics

Carbimazole is absorbed orally and is metabolized in the liver to methimazole, which is the active form responsible for its therapeutic effects. The half-life of carbimazole can be prolonged in patients with hepatic impairment. The drug is present in breast milk, but this generally does not contraindicate breastfeeding, provided that neonatal development is monitored closely.

Contra-indications

• Severe blood disorders
• Bone marrow disorders
• A history of acute pancreatitis associated with previous treatment

Adverse effects

• Neutropenia
• Agranulocytosis
• Severe cutaneous adverse reactions (SCARs)
• Haemolytic anaemia
• Alopecia
• Nausea
• Headache
• Skin reactions
• Salivary gland enlargement
• Taste loss
• Malaise
• Myopathy
• Hepatic disorders
• Pancreatitis

Interactions

• Metyrapone (severe interaction - decreases effects)
• Digoxin (moderate interaction - affects concentration)

Precautions

• Use with caution in hepatic impairment
• Monitor white blood cell count regularly
• Consider benefit-risk assessment for use in pregnancy
• Breastfeeding should only continue if neonatal development is monitored

Pregnancy

Consider use only after a thorough benefit-risk assessment. Carbimazole is associated with an increased risk of congenital malformations.

Breast-feeding

Present in breast milk but does not preclude breastfeeding as long as neonatal development is closely monitored and the lowest effective dose is used.

Storage

Store in a cool, dry place, away from direct sunlight.

Formulations

• Carbimazole 5 mg tablets
• Carbimazole 10 mg tablets
• Carbimazole 15 mg tablets
• Carbimazole 20 mg tablets
• Carbimazole oral suspension

Mechanism of action

Carbimazole is an aitithyroid agent that decreases the uptake and concentration of inorganic iodine by thyroid, it also reduces the formation of di-iodotyrosine and thyroxine. Once converted to its active form of methimazole, it prevents the thyroid peroxidase enzyme from coupling and iodinating the tyrosine residues on thyroglobulin, hence reducing the production of the thyroid hormones T3 and T4.

Pharmacodynamics

Carbimazole is a carbethoxy derivative of methimazole. Its antithyroid action is due to its conversion to methimazole after absorption. It is used to treat hyperthyroidism and thyrotoxicosis.