Thiamazole (Methimazole): Thionamide for Hyperthyroidism
Thiamazole (Methimazole/Tapazole; in some countries carbimazole — a prodrug) inhibits thyroid hormone synthesis by blocking thyroid peroxidase (TPO), the enzyme that oxidises iodide and incorporates iodine into tyrosine residues to form T3 and T4.
First-line medical treatment for hyperthyroidism (Graves' disease, toxic multinodular goitre, toxic adenoma). Preferred over propylthiouracil (PTU) except in first trimester pregnancy and thyroid storm.
Mechanism of Action
Inhibits thyroid peroxidase (TPO), blocking the oxidation of iodide and organification of iodine (Wolff-Chaikoff effect not related). This blocks synthesis of thyroxine (T4) and triiodothyronine (T3). Has no effect on stored hormones or previously released thyroid hormone — onset delayed 2–6 weeks.
Indications & Use
Hyperthyroidism due to Graves' disease, toxic nodular goitre, or toxic adenoma. Preparation before thyroid surgery or radioiodine treatment. Thyroid storm (high doses). First trimester pregnancy (if antithyroid drug needed — less teratogenic than PTU in 2nd/3rd trimester).
Dosage
Initial: 20–40 mg/day (single daily dose, more convenient than PTU). Maintenance (once euthyroid): 5–10 mg/day. Thyroid storm: 60–80 mg/day. Monitor TSH and free T4 every 4–6 weeks. Titrate to normal thyroid function.
Side Effects
Common: rash, pruritus, arthralgia (5–10%). Serious: agranulocytosis (0.1–0.5% — potentially fatal — educate patients to report fever/sore throat immediately). Rare: hepatotoxicity, ANCA-associated vasculitis. Antithyroid effect if overdosed.
Drug Interactions
Warfarin: hyperthyroidism increases warfarin catabolism — achieving euthyroidism may increase warfarin effect (bleeding risk). Beta-blockers: given for thyroid storm symptom control — reduce dose as euthyroidism achieved. Iodine-containing drugs (amiodarone): complex interactions with thyroid function.
Contraindications
Agranulocytosis on previous thionamide treatment. Severe hepatic disease. Pregnancy (2nd/3rd trimester — prefer PTU; thiamazole in 1st trimester to avoid PTU hepatotoxicity). Breastfeeding: low doses usually acceptable (monitor infant).
Frequently Asked Questions
What is the agranulocytosis risk with thiamazole?
Agranulocytosis occurs in 0.1–0.5% of patients, typically within the first 90 days. It is potentially fatal. Patients must be educated to contact their doctor immediately if they develop fever, sore throat, or mouth sores. Do not restart thiamazole after agranulocytosis — switch to another treatment (radioiodine, surgery).
Why is PTU preferred in first trimester pregnancy?
Thiamazole is associated with aplasia cutis (scalp defect) and choanal/oesophageal atresia in first trimester exposure. PTU is preferred in the 1st trimester. In 2nd/3rd trimester, thiamazole is preferred (PTU has higher hepatotoxicity risk). This switching approach is recommended by endocrine guidelines.
How long is antithyroid drug treatment continued?
For Graves' disease: typically 12–18 months of treatment. After stopping, remission rate is ~50% at 1 year. Predictors of remission: small goitre, low TRAb titre, mild hyperthyroidism. Recurrence is managed with further courses, radioiodine, or thyroidectomy.
References
- EMA Thyrozol SPC 2023
- ATA Hyperthyroidism Guidelines 2016
- ETA Guidelines Hyperthyroidism 2022
Medical Disclaimer: This information is for educational purposes only and does not replace professional medical advice.