Oral Adjunct to radio-iodine therapy, Hyperthyroidism, Preparation for thyroidectomy
Adult: Initially, 15-60 mg daily in 2-3 divided doses, titrate dose against thyroid function until euthyroidism is achieved. Maintenance: 5-15 mg daily (may be given as a single dose), adjust dose as needed to maintain euthyroid state. Blocking-replacement regimen: Initially, 20-60 mg daily in combination with levothyroxine. Treatment duration: 6-18 months. Child: 3-17 years Initially, 15 mg daily adjusted according to response.
Hepatic Impairment
Severe: Contraindicated.
Administration
May be taken with or without food. Take consistently w/ or w/o meals.
Contraindications
Serious, pre-existing haematological conditions; history of acute pancreatitis (after carbimazole or thiamazole administration). Severe hepatic impairment.
Special Precautions
Patient with intrathoracic goitre. Temporarily stop carbimazole therapy at the time of radio-iodine administration. Renal and mild to moderate hepatic impairment. Children and elderly. Pregnancy and lactation.
Adverse Reactions
Significant: Hypoprothrombinaemia, bleeding; haematologic effects or bone marrow depression (e.g. neutropenia, eosinophilia, leucopenia); acute pancreatitis. Rarely, severe dermatologic reactions (e.g. cutaneous vasculitis, erythema nodosum, dermatomyositis); hepatic reactions (e.g. cholestatic hepatitis, jaundice). Blood and lymphatic system disorders: Rarely, pancytopenia, aplastic anaemia, haemolytic anaemia, thrombocytopenia. Endocrine disorders: Insulin autoimmune syndrome. Gastrointestinal disorders: Nausea, mild gastrointestinal disturbance, loss of sense of taste, acute salivary gland swelling. General disorders and administration site conditions: Fever, malaise. Immune system disorders: Angioedema, multisystem hypersensitivity reactions. Investigations: Abnormal LFT. Musculoskeletal and connective tissue disorders: Arthralgia, myopathy. Nervous system disorders: Headache, neuritis, polyneuropathy. Skin and subcutaneous tissue disorders: Rash, pruritus, urticaria, hair loss. Rarely, Stevens-Johnson syndrome. Potentially Fatal: Agranulocytosis.
Perform confirmatory test for hyperthyroidism prior to initiation of therapy. Monitor CBC with differential, LFT at baseline and when clinically indicated; prothrombin time periodically and prior to surgery; creatine kinase as clinically indicated. Monitor thyroid function 2-6 weeks after initiation of therapy, repeat in 4-6 weeks if dose is adjusted, then every 2-3 months once euthyroid is achieved. Monitor TSH periodically. Assess for signs and symptoms of illness (e.g. sore throat, bruising or bleeding, mouth ulcers, fever, malaise).
Drug Interactions
May have cross-sensitivity with propylthiouracil (PTU) or thiamazole. May enhance the effect of anticoagulants. May increase the serum levels of theophylline which may result to toxicity. May increase the clearance of prednisolone. May reduce the clearance of erythromycin.
Action
Description: Mechanism of Action: Carbimazole, a prodrug, is metabolised into thiamazole (also known as methimazole) which is responsible for its antithyroid action. It inhibits the organification of iodide and coupling of iodothyronine residues leading to the suppression of thyroid hormone synthesis. Onset: 12-18 hours (thiamazole). Duration: 36-72 hours (thiamazole). Pharmacokinetics: Absorption: Rapidly absorbed from the gastrointestinal tract. Time to peak plasma concentration: Approx 1-2 hours (thiamazole). Distribution: Concentrated in the thyroid gland (thiamazole). Crosses the placenta and enters breast milk. Volume of distribution: 0.5 L/kg (thiamazole). Plasma protein binding: Moderately bound (thiamazole). Metabolism: Rapidly and completely metabolised in the liver into thiamazole (active metabolite); undergoes enterohepatic recirculation. Excretion: Via urine (approx 90%, as thiamazole or its metabolite); faeces (10%). Elimination half-life: 5.3-5.4 hours (carbimazole); approx 3-6 hours (thiamazole).
Chemical Structure
Carbimazole Source: National Center for Biotechnology Information. PubChem Compound Summary for CID 31072, Carbimazole. https://pubchem.ncbi.nlm.nih.gov/compound/Carbimazole. Accessed June 28, 2022.
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