What is the first‑line medication and starting dose for a non‑pregnant adult woman with newly diagnosed mild hyperthyroidism?

First-Line Medication for Mild Hyperthyroidism in Non-Pregnant Women

For this non-pregnant woman with mild hyperthyroidism, methimazole is the preferred first-line medication at a starting dose of 15 mg daily, divided into three doses of 5 mg every 8 hours. 1

Rationale for Methimazole Selection

• Methimazole is the drug of choice for hyperthyroidism in non-pregnant adults because of its longer half-life, once-daily dosing capability (though FDA labeling suggests divided doses), better efficacy, and fewer severe side effects compared to propylthiouracil 2
• Propylthiouracil is specifically reserved for first-trimester pregnancy due to methimazole's potential teratogenic effects in that specific context 3, 4, 5
• Since the pregnancy test is negative and the patient is not pregnant, the concerns about first-trimester teratogenicity with methimazole do not apply 4

Specific Prescription Details

Prescription:

• Medication: Methimazole 5 mg tablets
• Dose: 5 mg orally three times daily (every 8 hours)
• Total daily dose: 15 mg/day
• Duration: 15-day supply (45 tablets total)
• Instructions: Take one 5 mg tablet three times daily at approximately 8-hour intervals 1

Dosing Algorithm by Disease Severity

The FDA-approved dosing for methimazole is stratified by hyperthyroidism severity 1:

• Mild hyperthyroidism: 15 mg daily (this patient's category)
• Moderately severe hyperthyroidism: 30-40 mg daily
• Severe hyperthyroidism: 60 mg daily
• All doses should be divided into three doses at 8-hour intervals 1

Monitoring and Follow-Up Plan

• Monitor free T4 or free thyroxine index (FTI) every 2-4 weeks to guide dosage adjustments and ensure thyroid hormone levels remain in the high-normal range 4, 5
• The goal is to use the lowest possible thioamide dosage that maintains euthyroidism 4, 5
• Maintenance dosing typically ranges from 5-15 mg daily once thyroid levels stabilize 1

Critical Safety Monitoring

Watch for serious adverse effects that require immediate attention:

• Agranulocytosis: Monitor for sore throat, fever, or signs of infection 4, 5
• Hepatotoxicity: Monitor liver function, particularly with propylthiouracil (less common with methimazole) 4
• Other rare complications: Vasculitis and thrombocytopenia 4

Important Counseling Points

• If the patient becomes pregnant while on methimazole, she should contact you immediately to switch to propylthiouracil for the first trimester, then potentially back to methimazole for the second and third trimesters 3, 4, 5
• Untreated hyperthyroidism carries significant risks including heart failure, preeclampsia (if pregnant), and preterm delivery 4, 5
• Symptom control with beta-blockers (such as propranolol) can be added temporarily if she has significant tremors or palpitations while waiting for methimazole to take effect 4, 5

Common Pitfall to Avoid

Do not use propylthiouracil as first-line therapy in non-pregnant patients due to its association with more severe hepatotoxicity and the need for more frequent dosing 3, 4, 6. Propylthiouracil should be reserved specifically for first-trimester pregnancy or patients who cannot tolerate methimazole 4, 5, 2.