How to Take Antithyroid Drugs
Medication Selection and Dosing
Methimazole is the preferred antithyroid drug for most patients with hyperthyroidism due to its superior efficacy, safety profile, and convenient once-daily dosing. 1, 2, 3
Methimazole Dosing
- Initial dose: 15 mg daily for mild hyperthyroidism, 30-40 mg daily for moderate hyperthyroidism, and 60 mg daily for severe hyperthyroidism 2
- Administration: Take as a single daily dose or divided into 3 doses at 8-hour intervals 2
- Maintenance dose: 5-15 mg daily once euthyroid state is achieved 2
Propylthiouracil (PTU) Dosing
- Reserved for: First trimester of pregnancy or patients intolerant to methimazole 1
- Initial dose: 300 mg daily in divided doses; may increase to 400 mg daily for severe cases (occasionally 600-900 mg daily) 4
- Administration: Must be divided into 3 equal doses at 8-hour intervals 4
- Maintenance dose: 100-150 mg daily 4
Methimazole is 10 times more potent than PTU and achieves euthyroidism significantly faster—a single daily 15 mg dose of methimazole is more effective than 150 mg of PTU. 5
Timing and Administration
When to Take
- Antithyroid drugs can be taken with or without food 2, 4
- Consistency is key: Take at the same time(s) each day to maintain stable drug levels
- For divided doses, space approximately 8 hours apart 2, 4
Duration of Treatment
- Long-term therapy: 12-18 months for Graves' disease with goal of inducing remission 1, 6
- Short-term therapy: Weeks to months when preparing for radioiodine or surgery 1, 6
- Toxic nodular goiter: Antithyroid drugs will not cure this condition; definitive therapy (radioiodine or surgery) is required 6
Monitoring Requirements
Thyroid Function Testing
Monitor free T4 or free T3 (not TSH) every 2-4 weeks during initial treatment to guide dose adjustments. 1
- Target: Maintain free T4/T3 in the high-normal range using the lowest effective dose 1
- TSH remains suppressed for months even after achieving euthyroidism, so do not use TSH to guide early dose adjustments 1
- Once stable, continue monitoring every 2-3 months
Critical Safety Monitoring
Agranulocytosis occurs in 0.1-0.5% of patients, typically within the first 3 months of treatment. 1, 7
Warning Signs Requiring Immediate Medical Attention:
- Sore throat and fever (agranulocytosis) 1
- Fever, nausea, vomiting, right upper quadrant pain, dark urine, jaundice (hepatotoxicity, especially with PTU) 1
- Skin changes, blood in urine, respiratory symptoms (vasculitis) 1
If any of these symptoms develop, stop the medication immediately and obtain a complete blood count (CBC). 1
Adjunctive Symptomatic Management
Beta-blockers provide immediate relief of hyperthyroid symptoms while waiting for antithyroid drugs to take effect. 1
- Atenolol: 25-50 mg daily, or 1
- Propranolol: Dose varies based on symptoms 1
- Target: Heart rate <90 bpm if blood pressure allows 1
- Reduce dose once euthyroid state is achieved 1
Special Populations
Pregnancy
- First trimester: Use PTU due to risk of methimazole-associated birth defects (aplasia cutis, choanal/esophageal atresia) 1, 8
- After first trimester: Switch to methimazole 1
- Goal: Maintain free T4/T3 in high-normal range using lowest possible dose 1
- Both drugs are compatible with breastfeeding 1, 8
Elderly Patients
- Start with lower doses due to increased risk of cardiac complications 4
- Monitor more closely for adverse effects 4
Common Pitfalls to Avoid
- Do not reduce methimazole dose based solely on suppressed TSH while free T4 remains elevated—this leads to inadequate treatment 1
- Do not use antithyroid drugs for destructive thyroiditis—this is self-limited and requires only beta-blockers for symptom relief 1
- Never attempt cardioversion in thyrotoxic atrial fibrillation without first achieving euthyroid state 1
- Avoid pregnancy for 4 months after radioiodine therapy 1, 6