Methimazole 10-20 mg daily
For a patient with moderate hyperthyroidism, initiate methimazole at 10-20 mg once daily, with the specific dose within this range determined by the severity of biochemical hyperthyroidism and goiter size. 1
Dosing Algorithm
Initial dose selection:
- Standard moderate hyperthyroidism (free T4 <2-3x upper limit of normal, no large goiter): Start with 10-15 mg daily
- More severe presentation (free T4 2-3x upper limit, larger goiter, more symptomatic): Start with 15-20 mg daily
- All doses given once daily - methimazole has sufficient duration of action for single daily dosing, which improves adherence 1
Maintenance titration: Once thyroid function normalizes (typically 4-8 weeks), reduce to maintenance dose of 5-10 mg daily 1. Avoid "block and replace" regimens (continuing high-dose methimazole plus levothyroxine) as this increases adverse effects without improving outcomes 1.
Why Methimazole Over Propylthiouracil
Methimazole should be used in virtually every patient choosing antithyroid drug therapy 1. The only exceptions are:
- First trimester pregnancy (use PTU)
- Thyroid storm
- Minor reactions to methimazole in patients refusing radioactive iodine or surgery
Critical safety consideration: PTU carries unacceptable hepatotoxicity risk, including fulminant hepatic necrosis requiring liver transplantation or causing death. The FDA issued safety alerts after 32 cases (22 adult, 10 pediatric) of serious liver injury 1. Methimazole has significantly lower risk of major adverse effects 1.
Mandatory Pre-Treatment Steps
Before initiating methimazole 1:
- Obtain baseline labs: CBC with differential, liver function tests (bilirubin, transaminases)
- Patient education on warning signs requiring immediate drug cessation and physician contact:
- Fever, pharyngitis, mouth sores (agranulocytosis)
- Jaundice, dark urine, pale stools, right upper quadrant pain (hepatotoxicity)
- Severe rash, arthralgias
Adjunctive Therapy
Add beta-blocker if the patient has:
- Resting heart rate >90 bpm
- Cardiovascular disease
- Significant symptoms (tremor, palpitations, anxiety)
Propranolol 20-40 mg three times daily or atenolol 25-50 mg daily are typical starting doses 1.
Common Pitfall
Do not use excessive initial doses. Starting doses >20 mg daily increase risk of dose-dependent agranulocytosis without improving time to euthyroidism 2. The goal is to render the patient euthyroid "as quickly and safely as possible," not to maximally suppress thyroid function 1.