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How does propylthiouracil work?

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on Oct 19, 2021.

Official answer


Propylthiouracil (PTU) works by blocking the production of the thyroid hormones thyroxine (T4) and triiodothyronine (T3) in the thyroid gland. It does this by interfering with the thyroid peroxidase enzyme, which normally aids in the incorporation of iodine into the thyroglobulin protein to form the thyroid hormones.

Propylthiouracil also blocks the conversion of thyroxine (T4) to triiodothyronine (T3) in peripheral tissues and may be an effective treatment for thyroid storm. It's mechanism of action does not interfere with the effectiveness of thyroid hormones given by mouth or injection, or inactivate existing T4 and T3 in the blood and thyroid gland.

Propylthiouracil is classified as an oral prescription antithyroid medicine. Thyroid hormones regulate the body’s metabolism to store and use energy. Antithyroid medications are used to treat hyperthyroidism (overactive thyroid), Grave’s disease and toxic goiter. Other ways to treat hyperthyroidism is with surgery or radioactive iodine.

Clinically, methimazole (Tapazole) is usually the drug of choice over propylthiouracil in patients with hyperthyroidism because of easier dosing and less chance of serious side effects such as liver toxicity.

The propylthiouracil package insert carries a Boxed Warning that states:

  • Propylthiouracil should be reserved for patients with hyperthyroidism who cannot tolerate methimazole and in whom radioactive iodine therapy or surgery are not appropriate treatments.
  • Propylthiouracil may be the treatment of choice when an antithyroid drug is indicated during or just prior to the first trimester of pregnancy.
  • Severe liver injury and acute liver failure, in some cases requiring liver transplantation or resulting in death, have been reported in adult and pediatric patients.

What does PTU treat?

Propylthiouracil (PTU) is an oral medicine used:

  • in patients with Graves’ disease with hyperthyroidism (overactive thyroid) or toxic goiter (enlarged thyroid) who are intolerant of methimazole and for whom surgery or radioactive iodine therapy is not an appropriate treatment option
  • before certain thyroid surgeries or treatment with radioactive iodine in patients who do not tolerate methimazole.
  • Propylthiouracil is not recommended for use in children.

What do thyroid hormones do?

Thyroid hormones help control body temperature, heart rate, growth, and weight. When hyperthyroidism occurs and there are more thyroid hormones than needed, the body's metabolism is increased, which can lead to a variety of symptoms such as:

  • anxiety, trouble sleeping restlessness and tremors
  • fatigue, weakness
  • weight loss
  • increased appetite
  • increased sweating and heat intolerance
  • irregular heartbeats
  • a lump in neck, bulging eyes
  • changes in bowel movements
  • altered menstrual cycles in women
  • breast enlargement or erectile dysfunction in men

How do I know if propylthiouracil is working?

You will undergo thyroid function tests in the laboratory every 4 to 8 weeks while on propylthiouracil (PTU) until your hyperthyroidism is under control, then every 2 to 3 months. Using a blood test, your doctor will monitor your free T4, T3 and thyroid stimulating hormone (TSH) levels during treatment.

How long does it take to work?

Significant improvement and normal thyroid function generally occurs following 4 to 12 weeks of therapy. It is important that you are kept in the euthyroid state (normal thyroid function) to prevent symptoms of hyperthyroidism.

You may remain on treatment for 12 to 18 months for Graves’ disease or toxic goiter, but exact lengths of treatment are not well established. Your doctor will monitor your progress.

Being hyperthyroid can make you feel shaky with tremors, fast heart beat, anxious, hot and tired. Your doctor may prescribe a beta-blocker medicine (for example, propranolol) to help reduce your symptoms, especially the fast heart rate, until the propylthiouracil takes full effect. Beta-blockers are heart medicines often used to treat angina (chest pain), control abnormal heart rhythms and to reduce high blood pressure.

This is not all the information you need to know about propylthiouracil for safe and effective use and does not take the place of talking to your doctor about your treatment. Review the full propylthiouracil information here, and discuss this information and any questions you have with your doctor or other health care provider.


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