Thiamazole

Antithyroid preparations. Sulfur-containing imidazole derivatives

ATC code: H03BB02

Brand names: Tapazole, Methimazole

Updated 2 Jun 2026 · Methodology

Indications

Graves disease

First line

Thiamazole is first-line medical therapy for Graves disease in non-pregnant adults. 2016 recommends thiamazole as the preferred thionamide (exceptions: first trimester of pregnancy and life-threatening thyroid storm). Starting dose 10–40 mg daily depending on severity, course duration 12–18 months. Remission is achieved in 40–50% of patients. On relapse, radioiodine therapy or thyroidectomy is considered.

Sources

ICD-10: E05.0 · ICD-11: 5A02.0

Pre-thyroidectomy preparation

First line

Thiamazole is prescribed 4–8 weeks before elective thyroidectomy to achieve euthyroidism. Goal: reduce intra- and postoperative thyroid storm risk. 7–10 days before surgery, Lugol's solution or potassium iodide is added to reduce thyroid vascularity.

Sources

Toxic adenoma

First line

Thiamazole is used to control thyrotoxicosis in toxic adenoma or toxic multinodular goiter before definitive treatment (radioiodine therapy or thyroidectomy). Long-term medical therapy alone does not provide durable remission in autonomously functioning nodules, so definitive treatment is discussed early.

Sources

ICD-10: E05.1 · ICD-11: 5A02.1

Marketing claims without evidence base

The drug is promoted for these uses outside international guidelines. Each entry below is analyzed against AEMPS, FDA, EMA, Cochrane and major RCTs.

Hyperthyroidism in the first trimester of pregnancy

Not recommended

In the first trimester, thiamazole is not used as first-line therapy. 2017 recommends switching to propylthiouracil at confirmation of pregnancy and continuing it through the first trimester. This is due to the risk of thiamazole embryopathy: aplasia cutis of the scalp, choanal and esophageal atresia, anterior abdominal wall defects. After the first trimester, a switch back to thiamazole is acceptable, as propylthiouracil carries higher hepatotoxicity risk.

Sources

ATA: 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum (2017)

ICD-10: O99.2 · ICD-11: JB64.2

How to read tiers A–F

AStrong evidence – first or second line in international guidelines
BRecommended in selected cases
CStudy data contradict each other
DBenefit not proven, no documented harm
FDocumented harm, or risk without benefit

Mechanism of action

Inhibits thyroid peroxidase – the key enzyme in thyroid hormone synthesis. This blocks tyrosine iodination and formation of thyroxine (T4) and triiodothyronine (T3) in the thyroid. Does not suppress release of previously synthesized hormones – the clinical effect develops as stores deplete, typically over 2–4 weeks. Does not affect peripheral T4-to-T3 conversion (unlike propylthiouracil).

Safety

Boxed warning

No formal FDA boxed warning in the US, but labels explicitly highlight the risk of fatal agranulocytosis and the need to stop the drug immediately on fever, sore throat or mouth ulcers.

Contraindications

Severe leukopenia, history of agranulocytosis
Severe hepatic impairment
Hypersensitivity to thiamazole or imidazole derivatives
First trimester of pregnancy (propylthiouracil preferred)
History of cholestatic jaundice on thionamides

Serious adverse effects

Agranulocytosis (0.2–0.5%, peaks in the first 3 months of therapy)
Acute hepatic failure
Cholestatic hepatitis
ANCA-positive vasculitis
Aplastic anemia

Common adverse effects

Rash, pruritus, urticaria
Arthralgia
Nausea, epigastric discomfort
Metallic taste

Uncommon adverse effects

Leukopenia, thrombocytopenia
ALT/AST increase
Alopecia
Polyneuritis

Pregnancy: can you take Thiamazole?

AEMPS summary of product characteristics (section 4.6): thiamazole is not the treatment of choice in pregnancy because it crosses the placenta and can cause fetal hypothyroidism and goitre; its risks are weighed against those of ongoing hyperthyroidism, which must be adequately treated to prevent serious maternal and fetal complications. Based on epidemiological studies and spontaneous reports, thiamazole is suspected of causing congenital malformations, particularly in the first trimester and at high doses: aplasia cutis congenita, craniofacial malformations (choanal atresia, facial dysmorphism), omphalocele, oesophageal atresia, omphalomesenteric duct anomaly and ventricular septal defect. It is given only after a strict individual benefit-risk assessment, at the lowest effective dose and without additional thyroid hormones, with close maternal, fetal and neonatal monitoring and frequent checks of the free thyroxine index. Fetal risks are higher near term, and thyroid dysfunction often eases during pregnancy, so the dose may need to be reduced: half the usual daily dose is given in the last trimester, and in some cases treatment can be stopped for the last 2–3 weeks. Women of childbearing potential should use effective contraception during treatment.

Breastfeeding: can you take Thiamazole?

Transfers into breast milk. ATA and LactMed allow breastfeeding at doses up to 20 mg daily, with dosing immediately after feeding. Infant TSH and free T4 are checked at 1 month.

Calculate RID Full profile on NIH LactMed ↗

Reference information, not a clinical decision. Discuss feeding pauses or changes with your physician or an IBCLC.

Practical notes

Monitoring

Before therapy and every 2–4 weeks during the first 3 months, monitor CBC with differential and liver enzymes (ALT, AST, ALP, bilirubin). Free T4 and T3 are checked 4–6 weeks after initiation, then every 2–3 months until euthyroidism. TSH may remain suppressed for months – do not rely on it alone early in treatment.

Timing and administration

Taken once daily in the morning or 2–3 times daily depending on starting dose. 4–8 weeks after reaching euthyroidism, switch to a maintenance dose of 5–10 mg daily or a «block-and-replace» regimen (thiamazole + levothyroxine).

Special situations

The patient receives detailed instructions: at onset of fever, sore throat, mouth ulcers or malaise, immediately stop the drug and obtain a CBC. This rule targets early detection of agranulocytosis, which occurs in 0.2–0.5% and peaks in the first 3 months. Onset of jaundice or dark urine warrants urgent evaluation with liver function tests.

Pregnancy

Women of reproductive age are counseled on thiamazole teratogenicity. When planning pregnancy, alternatives are discussed: radioiodine therapy (with at least 6 months before conception) or thyroidectomy. If pregnancy is confirmed on thiamazole, switch immediately to propylthiouracil for the first trimester.

Common myths

Myth: «thiamazole causes dependence». Fact: the drug does not cause dependence. The 12–18-month course reflects the immune nature of Graves disease, not drug properties.

Myth: «thiamazole cures hyperthyroidism permanently». Fact: it suppresses synthesis but does not eliminate the autoimmune mechanism. Remission is achieved in only 40–50%.

Myth: «thiamazole can be used in pregnancy as usual». Fact: in the first trimester it is replaced by propylthiouracil; later, switched back.

Label mentions7 mentions

Verbatim lines from the interaction sections, in the language of the label

From this drug’s label

Amiodarone
«La administración de amiodarona puede potenciar el efecto de los antitiroideos.»
AEMPS, section 4.5 · Open on CIMA
anticoagulantesclass named in the label
«El tiamazol puede disminuir el efecto de los anticoagulantes orales.»
AEMPS, section 4.5 · Open on CIMA
betabloqueantesclass named in the label
«El tiamazol puede potenciar los efectos de los siguientes fármacos: el propranolol u otros beta-bloqueantes (provocando broncoconstricción en hipertiroideos asmáticos o bronquíticos), los glucósidos…»
AEMPS, section 4.5 · Open on CIMA
digitálicosclass named in the label
«El tiamazol puede potenciar los efectos de los siguientes fármacos: el propranolol u otros beta-bloqueantes (provocando broncoconstricción en hipertiroideos asmáticos o bronquíticos), los glucósidos…»
AEMPS, section 4.5 · Open on CIMA
Glycerol
«La administración glicerol yodado, yodo o yoduro potásico puede disminuir la respuesta a los antitiroideos.»
AEMPS, section 4.5 · Open on CIMA
Propranolol
«El tiamazol puede potenciar los efectos de los siguientes fármacos: el propranolol u otros beta-bloqueantes (provocando broncoconstricción en hipertiroideos asmáticos o bronquíticos), los glucósidos…»
AEMPS, section 4.5 · Open on CIMA
Theophylline
«El tiamazol puede potenciar los efectos de los siguientes fármacos: el propranolol u otros beta-bloqueantes (provocando broncoconstricción en hipertiroideos asmáticos o bronquíticos), los glucósidos…»
AEMPS, section 4.5 · Open on CIMA

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Frequently asked

What is Thiamazole used for?

Thiamazole is evaluated for the following indications with varying evidence strength: Pre-thyroidectomy preparation (evidence tier A), Graves disease (evidence tier A), Toxic adenoma (evidence tier A). See the full indication matrix with dosing and citations above on this page.

What are the side effects of Thiamazole?

Common side effects of Thiamazole (≥ 1 in 100): Rash, pruritus, urticaria, Arthralgia, Nausea, epigastric discomfort, Metallic taste. See the Safety section for uncommon and serious reactions.

Is Thiamazole safe during pregnancy?

Is Thiamazole compatible with breastfeeding?

Transfers into breast milk. ATA and LactMed allow breastfeeding at doses up to 20 mg daily, with dosing immediately after feeding. Infant TSH and free T4 are checked at 1 month.

Who should not take Thiamazole?

Thiamazole is contraindicated in: Severe leukopenia, history of agranulocytosis; Severe hepatic impairment; Hypersensitivity to thiamazole or imidazole derivatives; First trimester of pregnancy (propylthiouracil preferred); History of cholestatic jaundice on thionamides. Full list in the Safety section.

Does Thiamazole carry an FDA boxed warning?

No formal FDA boxed warning in the US, but labels explicitly highlight the risk of fatal agranulocytosis and the need to stop the drug immediately on fever, sore throat or mouth ulcers.

thiamazole causes dependence

the drug does not cause dependence. The 12–18-month course reflects the immune nature of Graves disease, not drug properties.

thiamazole cures hyperthyroidism permanently

it suppresses synthesis but does not eliminate the autoimmune mechanism. Remission is achieved in only 40–50%.

thiamazole can be used in pregnancy as usual

in the first trimester it is replaced by propylthiouracil; later, switched back.

Official sources

Direct links to regulator labels. Open in a new tab.

AEMPS / CIMALabel dated 02/21/2020
GRLS

Other drugs in this group

Iodine (umbrella: potassium iodide, kelp / Fucus vesiculosus), Levothyroxine sodium, Potassium iodide