Propylthiouracil: Mechanism of Action, Dosage, Interactions, Side Effects, Precautions and Warnings

It is an oral medication that is used to control hyperthyroidismcaused by an overactive thyroid gland.

Propylthiouracil (PTU) is an antithyroid drug that has a mechanism of action similar to methimazole (Tapazol).

The most common cause of hypothyroidism is Graves’ disease.

It is an autoimmune disease in which an individual produces antibodies against the thyroid-stimulating hormone receptors in the cells of the thyroid gland and then triggers the excessive production of thyroid hormone by the cells.

Mechanism of action

The two thyroid hormones made by the thyroid gland, thyroxine (T4) and triiodothyronine (T3), are formed by combining iodine and a protein called thyroglobulin with the help of an enzyme called peroxidase.

PTU inhibits iodine and peroxidase from their normal interactions with thyroglobulin to form T4 and T3.

Thanks to this, it is possible to reduce the production of thyroid hormone. PTU also interferes with the conversion of T4 to T3 and, since T3 is more potent than T4, it also reduces the activity of thyroid hormones.


  • The initial dose of OCT in adults is 300 mg per day divided into 3 separate doses.
  • PTU is usually administered every eight hours.
  • Patients with large goiters may require initial doses of up to 900 mg per day.
  • The usual long-term adult dose after initial treatment is 100-150 mg per day.


PTU may increase the effect of oral anticoagulants, for example, warfarin (Coumadin).

Therefore, changes in the warfarin dose and monitoring the effects of warfarin on blood coagulation are necessary.

Hyperthyroidism may cause a greater elimination of beta blockers, for example, propranolol (Inderal, Inderal LA, Innopran XL).

Once the hyperthyroidism is reversed, the excretion of beta-blockers can return to normal and less beta-blocker will be needed to provide the same effect.

Blood levels may increase when hyperthyroidism is reversed in patients with a stable dose of digoxin .

A smaller dose of digoxin may be necessary to avoid the toxicity of digoxin.

Similarly, the elimination of theophylline may decrease when hyperthyroidism is reversed in patients with a stable dose of theophylline.

A reduced dose of theophylline may be necessary to avoid its toxicity.

Side effects

The most common side effects are related to the skin, such as:

  • Eruption.
  • Itch.
  • Urticaria.
  • Abnormal hair loss
  • Pigmentation of the skin.

Other important side effects include:

  • Acidity.
  • Numbness.
  • Nausea.
  • Lost of taste
  • Swelling.
  • Headache.
  • Vomiting
  • Articular or muscular pain.

Less common but serious side effects have occurred with PTU therapy.

There can be a decrease in white blood cells in the blood ( agranulocytosis ), as well as a reduction in red blood cells.

Symptoms and signs of agranulocytosis include infectious lesions of the throat, gastrointestinal tract, and skin with a general feeling of illness, unusual tiredness, persistent sore throat, bleeding, and fever, so if you experience any of these symptoms, you should contact As soon as possible with your doctor.

These symptoms usually appear during the first months of treatment.

A decrease in blood platelets ( thrombocytopenia ) may also occur .

Thrombocytopenia can cause excessive bleeding, because platelets are important for blood clotting.

Severe hepatic injury and acute liver failure, in some fatal cases, have been associated with PTU.

Some adults and pediatric patients may require a liver transplant.

Stomach upset, nausea, vomiting, rash, mild itching, or headache may occur.

Tell your doctor right away if you have rare but serious pain in the muscle and joints or signs of kidney problems (such as a change in the amount of urine).

A very serious allergic reaction is rare after taking this medicine.

However, you should get immediate medical attention if symptoms of an allergic reaction occur, such as: severe dizziness, difficulty breathing, swelling (especially of the face, throat and tongue), rash and itching.

Call your doctor or pharmacist if any of these symptoms gets worse or persists.

In general, people who take PTU do not have side effects.

Precautions and warnings

In pregnancy propylthiouracil crosses the placenta and can cause damage to the fetus.

If it is necessary to use OCT during pregnancy, the lowest effective dose should be used.

Since methimazole is associated with fetal abnormalities, PTU is used during the first trimester if an antithyroid drug is needed.

Propylthiouracil is excreted in breast milk in small amounts.

If you are allergic to propylthiouracil, any of the components of propylthiouracil or any other medication, tell your doctor and pharmacist.

For a better understanding of propylthiouracil ingredients, ask your doctor or pharmacist to consult the medication guide.

If you are taking or plan to take other prescription or over-the-counter medications, herbal products, nutritional supplements or vitamins, let your doctor know for a correct evaluation.

If you suffer or have ever suffered from: aplastic anemia (condition in which the body does not produce enough new blood cells), thrombocytopenia (decrease in platelets), leukopenia (decrease in white blood cells) or some other disease that affects the production of white blood cells, platelets or red blood cells, you should inform your doctor.