What is the typical starting dose of Thyrozol (Methimazole) for adults with hyperthyroidism?

Methimazole (Thyrozol) Dosage for Hyperthyroidism

The initial daily dosage of Thyrozol (methimazole) for adults with hyperthyroidism is 15 mg for mild hyperthyroidism, 30-40 mg for moderately severe hyperthyroidism, and 60 mg for severe hyperthyroidism, typically divided into 3 doses at 8-hour intervals. 1

Initial Dosing Strategy

The FDA-approved dosing guidelines for methimazole are clear and should be followed based on disease severity:

• Mild hyperthyroidism: 15 mg daily
• Moderate hyperthyroidism: 30-40 mg daily
• Severe hyperthyroidism: 60 mg daily

These doses are traditionally divided into three equal portions taken approximately every 8 hours 1.

Administration Options

While the standard recommendation is to divide the daily dose into three equal portions, research evidence suggests that a single daily dose may be equally effective:

• Studies have demonstrated that a single daily dose of 15 mg methimazole is as effective as divided doses in achieving euthyroidism 2, 3
• The mean time to achieve euthyroidism is similar between single daily dosing (5.8 ± 3.1 weeks) and divided dosing (5.6 ± 2.7 weeks) 3
• Single daily dosing may be associated with fewer adverse effects (13% vs 24% with divided dosing) 3

Maintenance Dosing

After achieving euthyroidism:

• Reduce to a maintenance dose of 5-15 mg daily 1
• Long-term studies show maintenance doses can be as low as 5 mg daily 3

Monitoring and Dose Adjustments

• Monitor thyroid function (TSH and free T4) every 4-6 weeks initially 4
• Once stable, monitor every 6-12 months 4
• Adjust dose based on clinical response and laboratory values
• Watch for signs of overtreatment (hypothyroidism): fatigue, cold intolerance, weight gain, dry skin, constipation 4

Special Considerations

• Elderly patients or those with cardiac disease: Consider starting with lower doses and titrating slowly 4
• Pregnancy: Methimazole crosses the placenta and requires special consideration; some guidelines suggest avoiding methimazole during pregnancy 5

Common Pitfalls to Avoid

1. Overtreatment: Elevated TSH with low free T4 suggests iatrogenic hypothyroidism requiring dose reduction 4
2. Inadequate monitoring: Failure to monitor thyroid function regularly can lead to under or overtreatment
3. Medication interactions: Be aware that estrogens, oral contraceptives, proton pump inhibitors, calcium and iron supplements can affect thyroid medication metabolism 4
4. Non-adherence: Single daily dosing may improve medication adherence compared to multiple daily doses

By following these guidelines, clinicians can effectively manage hyperthyroidism while minimizing the risk of adverse effects and optimizing patient outcomes.