Carbimazole

Antithyroid Agent·Endocrinology - Thyroid & Bone

Hyperthyroidism, preparation for thyroidectomy or radioiodine therapy

PBSTHYROIDNeo-Mercazole5 mg tabletS4+1

Max Dose

60 mg/day

Half-life

6 h

Neutropenia

CRITICAL

Rash

COMMON

Rapid Summary

Overview

Thionamide antithyroid drug. The first-line medical therapy for Graves' disease and hyperthyroidism. Suppresses thyroid hormone synthesis.

Maximum Dose

Max 60 mg/day (During severe acute thyrotoxicosis). Maintenance is typically 5-15 mg/day.

Clinical Focus

Carries a rare but life-threatening risk of agranulocytosis. Patients must present to ED immediately if they develop a sore throat or fever.

Bottom Line

Carbimazole is the first-line antithyroid drug for Graves' disease and thyrotoxicosis, but carries rare risk of fatal agranulocytosis requiring urgent FBC if sore throat/fever develops.

Formulation & Access

Brand Names

Neo-Mercazole

Oral Routes

Tablets (5 mg).

Prescribing & PBS

Unrestricted General Benefit (S4).

Indications & Use

Primary

PBSTGA

Hyperthyroidism, preparation for thyroidectomy or radioiodine therapy.

Clinical Place

First-line for Graves' disease. Preferred over propylthiouracil (PTU) due to lower risk of severe hepatotoxicity.

Quick reference

Class & Role

Antithyroid Agent — Thionamide antithyroid drug.

Route / Formulation

Tablets (5 mg). (Neo-Mercazole)

Usual Dose & Max

PO 15-40 mg/day (given as a single dose or divided BD). Max 60 mg/day (During severe acute thyrotoxicosis). Maintenance is typically 5-15 mg/day.

Key Indication Doses

Hyperthyroidism (Initiation): PO 15-40 mg/day (given as a single dose or divided BD). Maintenance Therapy: PO 5-15 mg OD. Titrated strictly based on TFTs every 4-6 weeks. Block and Replace Regimen: PO 40 mg OD (to completely shut down the thyroid), co-administered with Levothyroxine 50-150 mcg OD to maintain euthyroidism.

Best Uses

Carbimazole is the first-line antithyroid drug for Graves' disease and thyrotoxicosis, but carries rare risk of fatal agranulocytosis requiring urgent FBC if sore throat/fever develops.

Avoid / Cautions

History of carbimazole-induced agranulocytosis or severe liver injury. Pregnancy Category C. PTU is generally preferred in the 1st trimester (due to rare carbimazole-associated embryopathy), but carbimazole is often used in 2nd/3rd trimesters.

Key Risks

Haematological: Agranulocytosis (absolute neutrophil count < 0.5). Occurs in 0.5% of patients, usually within the first 3 months. Hepatic: Cholestatic jaundice (less common than with PTU, but still a risk). Dermatological: Pruritic maculopapular rash (occurs in ~5% of patients, often transient).

Key Interactions

As hyperthyroidism resolves, the clearance of other drugs (like Warfarin or Digoxin) slows down, massively increasing their toxicity if their doses aren't reduced accordingly.

Monitoring

Routine TFTs (TSH, Free T4, Free T3). FBC if patient reports any signs of infection. Baseline LFTs. Counsel patient: 'If you get a sore throat, mouth ulcers, or fever, stop the medication and go to the Emergency Department for a blood test immediately.'

Clinical Pearl

When treating hyperthyroidism, the Free T4 and Free T3 will normalize weeks before the TSH rises. Dose adjustments should initially be based on Free T4/T3, not TSH, to avoid over-treating and causing hypothyroidism.

Access

Neo-Mercazole5 mg tablet

Brand: Neo-Mercazole
Formulation: Tablets (5 mg)

Carbimazole

Antithyroid Agent·Endocrinology - Thyroid & Bone

Hyperthyroidism, preparation for thyroidectomy or radioiodine therapy

PBSTHYROIDNeo-Mercazole5 mg tabletS4+1

Max Dose

60 mg/day

Half-life

6 h

Neutropenia

CRITICAL

Rash

COMMON

Rapid Summary

Overview

Thionamide antithyroid drug. The first-line medical therapy for Graves' disease and hyperthyroidism. Suppresses thyroid hormone synthesis.

Maximum Dose

Max 60 mg/day (During severe acute thyrotoxicosis). Maintenance is typically 5-15 mg/day.

Clinical Focus

Carries a rare but life-threatening risk of agranulocytosis. Patients must present to ED immediately if they develop a sore throat or fever.

Bottom Line

Carbimazole is the first-line antithyroid drug for Graves' disease and thyrotoxicosis, but carries rare risk of fatal agranulocytosis requiring urgent FBC if sore throat/fever develops.

Formulation & Access

Brand Names

Neo-Mercazole

Oral Routes

Tablets (5 mg).

Prescribing & PBS

Unrestricted General Benefit (S4).

Indications & Use

Primary

PBSTGA

Hyperthyroidism, preparation for thyroidectomy or radioiodine therapy.

Clinical Place

First-line for Graves' disease. Preferred over propylthiouracil (PTU) due to lower risk of severe hepatotoxicity.

Quick reference

Class & Role

Antithyroid Agent — Thionamide antithyroid drug.

Route / Formulation

Tablets (5 mg). (Neo-Mercazole)

Usual Dose & Max

PO 15-40 mg/day (given as a single dose or divided BD). Max 60 mg/day (During severe acute thyrotoxicosis). Maintenance is typically 5-15 mg/day.

Key Indication Doses

Best Uses

Carbimazole is the first-line antithyroid drug for Graves' disease and thyrotoxicosis, but carries rare risk of fatal agranulocytosis requiring urgent FBC if sore throat/fever develops.

Avoid / Cautions

Key Risks

Key Interactions

As hyperthyroidism resolves, the clearance of other drugs (like Warfarin or Digoxin) slows down, massively increasing their toxicity if their doses aren't reduced accordingly.

Monitoring

Clinical Pearl

Access

Neo-Mercazole5 mg tablet

Brand: Neo-Mercazole
Formulation: Tablets (5 mg)