What first‑line medication and dosing are recommended for a newly diagnosed adult with hyperthyroidism, and what alternatives are used in pregnancy or thionamide intolerance?

Medication for Hyperthyroidism

Methimazole is the first-line antithyroid drug for virtually all newly diagnosed adults with hyperthyroidism, started at 10-20 mg daily and titrated to a maintenance dose of 5-10 mg daily, with the critical exception that propylthiouracil must be used during the first trimester of pregnancy. 1

First-Line Treatment for Non-Pregnant Adults

Methimazole (MMI) is strongly preferred over propylthiouracil (PTU) because it offers:

• Once-daily dosing (vs. 2-3 times daily for PTU)
• Lower risk of major side effects, particularly severe hepatotoxicity
• Superior efficacy in achieving euthyroidism 1

Initial Dosing Algorithm

For mild hyperthyroidism without large goiter:

• Start methimazole 10-20 mg once daily 1
• Titrate down to maintenance dose of 5-10 mg daily once euthyroid 1

For severe hyperthyroidism or large goiter:

• Start methimazole 30-40 mg daily 2
• Higher initial doses restore euthyroidism more rapidly

Treatment duration: 12-18 months minimum 2

Critical Pre-Treatment Requirements

Before starting any antithyroid drug, obtain 1:

• Complete blood count with differential
• Liver function tests (bilirubin and transaminases)

Pregnancy-Specific Management

The approach differs dramatically by trimester due to competing risks of teratogenicity versus hepatotoxicity:

First Trimester (Weeks 0-13)

Use propylthiouracil exclusively 1, 3

• PTU starting dose: 50-150 mg three times daily
• Maintenance: 50 mg two to three times daily
• Rationale: Methimazole causes embryopathy (aplasia cutis, choanal/esophageal atresia) 1

Second and Third Trimesters (Week 14 onward)

Switch to methimazole 1

• Use lowest dose to keep free T4 at or slightly above upper limit of normal range
• Keep TSH suppressed (this is acceptable in pregnancy) 1
• Monitor thyroid function monthly 1
• Rationale: PTU carries risk of potentially fatal hepatotoxicity that increases with duration of use 1

Conversion ratio when switching drugs:

• 300 mg PTU ≈ 10-15 mg methimazole (potency ratio 20-30:1) 1

Important caveat: For women already on methimazole who become pregnant, obtain pregnancy testing immediately and switch to PTU as soon as pregnancy is confirmed 1

Alternatives for Thionamide Intolerance

When patients cannot tolerate methimazole due to minor reactions (rash, arthralgias) and refuse radioactive iodine or surgery 1:

1. Switch to propylthiouracil (non-pregnant patients)

   • Only if minor reactions to methimazole
   • Monitor liver enzymes every 4 weeks due to hepatotoxicity risk

2. Definitive therapy becomes necessary:

   • Radioactive iodine ablation (preferred for most)
   • Thyroidectomy (if radioiodine contraindicated)

Critical Safety Monitoring

Patients must be instructed to STOP medication immediately and contact physician if they develop 1:

• Fever or pharyngitis (agranulocytosis warning)
• Jaundice, dark urine, or acolic stools (hepatotoxicity)
• Severe abdominal pain, nausea, or fatigue
• Pruritic rash or arthralgias

This warning must be reinforced at every visit 1.

Special Situations

Thyroid storm: PTU is preferred over methimazole due to additional peripheral T4 to T3 conversion blockade 1, though recent evidence suggests outcomes may be equivalent 4

Breastfeeding: Methimazole is preferred over PTU due to lower hepatotoxicity risk in both mother and infant 1

Block-and-replace therapy (high-dose antithyroid drug plus levothyroxine): Not recommended due to higher side effect rates 1

Common Pitfalls to Avoid

• Never use block-and-replace in pregnancy - it increases fetal hypothyroidism risk 1
• Don't rely on TSH alone in early treatment - it remains suppressed for weeks after free T4 normalizes 1
• Don't continue PTU beyond first trimester in pregnancy without compelling reason - hepatotoxicity risk increases with duration 1
• Don't undertreate pregnant patients - inadequately controlled hyperthyroidism causes worse fetal outcomes than appropriate antithyroid drug doses 3, 5