# Antithyroid Medications

URL: https://thethyroidologist.com/treatments/antithyroid-medications/
Section: Treatments
Last medically reviewed: 2025-11-01

> Antithyroid medications block thyroid hormone production. Methimazole and propylthiouracil — how they work, side effects, and monitoring.

Antithyroid drugs — primarily methimazole (also called thiamazole) and propylthiouracil (PTU) — block the thyroid’s ability to produce hormone.
They are the main medical treatment for hyperthyroidism, particularly Graves’ disease.

## How they work

Both drugs inhibit thyroid peroxidase — the enzyme that organifies iodine into hormone. Methimazole is generally preferred (once-daily dosing, better side-effect profile); PTU is used in specific situations including first-trimester pregnancy.
They reduce new hormone production but do not affect hormone already released — beta-blockers control symptoms meanwhile.

## Treatment course

For Graves’ disease, a typical course is 12–18 months, aiming for remission — some patients stay in remission after stopping; others relapse. Dosing is guided by thyroid function tests every 4–6 weeks initially.
Long-term low-dose therapy is an option for patients who cannot have RAI or surgery, though it carries ongoing side-effect considerations.

## Side effects

Common: mild rash, joint aches, taste disturbance. Serious but rare: agranulocytosis (dangerous drop in white cells — fever/sore throat needs urgent review), hepatitis, vasculitis. Any fever, sore throat, or unusual symptoms while on these drugs requires immediate medical attention.
Baseline and periodic blood counts may be monitored depending on the regimen.

## Frequently asked questions

### How long does it take for antithyroid drugs to work?

Hormone levels typically improve within weeks; full control may take 1–3 months. Beta-blockers manage symptoms during this period.

### Will I become hypothyroid on these drugs?

Possibly — if over-treated, thyroid function can fall too low. Dosing is carefully monitored to avoid this.

### Are these safe in pregnancy?

Both can be used in pregnancy with caution — PTU is preferred in the first trimester due to methimazole’s small risk of birth defects. Dosing requires specialist management.

## Disclaimer

The content on this website is for general educational purposes only and does not constitute medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional about your specific condition.

---
Part of The Thyroidologist — https://thethyroidologist.com