Drug reference

Carbimazole

Antithyroid (thionamide; prodrug of methimazole) · Hyperthyroidism

START

15–40 mg/day PO, titrate to euthyroid; maintenance 5–15 mg/day (or block-and-replace)

TYPICAL MAX

~40–60 mg/day initial (storm higher, specialist)

STOP IF

Agranulocytosis (fever/sore throat — urgent FBC, STOP), significant hepatotoxicity, vasculitis

WATCH

FBC if infection symptoms (agranulocytosis), LFTs, TFTs (titrate), counsel sore-throat warning

CDSCO approvedSchedule HATC H03BB01

Dose laddermg/d

Renal dose adjustmenteGFR mL/min/1.73m²

KDIGO 2024 + manufacturer label

Pharmacokineticsplasma · t hours

ONSET: 23min · absorption onset
PEAK: 1.5h · methimazole Cmax
t½: 5h · methimazole t½
DURATION: 1d · once-daily (intrathyroidal longer)

EXCRETION

Hepatic methimazole metabolism; renal metabolites

route + CYP

INTERACTIONS

—

none in our sources

PREGNANCY

Use lowest effective dose; first-trimester embryopathy risk (aplasia cutis, choanal/oesophageal atresia) — propylthiouracil preferred in 1st trimester then switch

FDA category + note

Available in India

20 branded formulations. Look up specific brands in the Drugs workspace.

Mechanism

Rapidly converted to methimazole, which inhibits thyroid peroxidase, blocking iodination/coupling of tyrosine residues and reducing thyroid hormone synthesis (not pre-formed hormone — effect over weeks).

Indications

Hyperthyroidism (Graves disease, toxic nodular goitre)Preparation for thyroidectomy or radioiodineThyroid storm (adjunct, higher doses)

Dosing

Adult: Initial 15–40 mg/day PO (divided or once daily), titrate to euthyroid then maintenance 5–15 mg/day; or 'block-and-replace' with levothyroxine. Thyroid storm: higher doses.
Pediatric: Initial ~0.5–0.75 mg/kg/day, titrate (specialist).
Renal adjustment: No specific adjustment.
Hepatic adjustment: Caution; reduce/withhold if hepatotoxicity (less hepatotoxic than propylthiouracil).
Geriatric: No specific adjustment; monitor counts.
Max dose: ~40–60 mg/day initial (thyroid storm higher, specialist)

Pharmacokinetics

Onset

Biochemical improvement 1–3 weeks; full effect weeks

Peak effect

Methimazole Cmax ~1–2 h

Duration

Once–twice daily (intrathyroidal action longer)

Half-life

Methimazole ~3–6 h (intrathyroidal effect prolonged)

Bioavailability

Rapidly/completely converted to methimazole

Protein binding

Low (methimazole)

Metabolism

Hepatic (methimazole)

Excretion

Renal (metabolites)

Contraindications

Severe pre-existing blood disorders (agranulocytosis/aplasia)
Prior serious adverse reaction to thionamides
Hypersensitivity to carbimazole/methimazole

Side effects

Common

Rash/pruritus, urticariaArthralgiaNausea, taste disturbanceMild transient leucopenia

Serious

Agranulocytosis (idiosyncratic — fever/sore throat → urgent FBC, stop)
Hepatotoxicity (cholestatic)
Aplastic anaemia/pancytopenia
Severe cutaneous reactions; ANCA-associated vasculitis
Acute pancreatitis (class)

Pregnancy & lactation

Pregnancy

Use lowest effective dose; first-trimester embryopathy risk (aplasia cutis, choanal/oesophageal atresia) — propylthiouracil preferred in 1st trimester then switch

Lactation

Compatible at low–moderate doses (≤20–30 mg/day) with infant thyroid monitoring; give after feeds