Why is propylthiouracil preferred in pregnancy?

Chloe Ramirez January 19th, 2026 at 10:57 PM

Propylthiouracil is the drug of choice for hyperthyroidism during pregnancy in North America owing to the suspected association of methimazole with congenital abnormalities (sometimes referred to as methimazole embryopathy), characterized by aplasia cutis, esophageal atresia, choanal atresia, facial abnormalities, and

Simply so, is it safe to take propylthiouracil while pregnant?

Both methimazole (MMI) and propylthiouracil (PTU) may be used during pregnancy; however, PTU is preferred in the first trimester and should be replaced by MMI after this trimester. Choanal and esophageal atresia of fetus in MMI-treated and maternal hepatotoxicity in PTU-treated pregnancies are of utmost concern.

Likewise, does PTU cross the placenta? Propylthiouracil (PTU) is widely believed to cross the placenta less freely than methimazole (MMI) and is therefore regarded as the preferred drug for treatment of hyperthyroidism in pregnancy. Clinical studies comparing the two drugs show, however, no differences in maternal or fetal thyroid function.

Also Know, does PTU cause birth defects?

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Background: Propylthiouracil (PTU) used in the treatment of maternal hyperthyroidism in early pregnancy may be associated with a higher prevalence of birth defects in the face and neck region and in the urinary system but the severity of these complications remains to be elucidated.

Does thyroid affect pregnancy baby?

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Early studies found that children born to mothers with severe, untreated hypothyroidism during pregnancy had lower IQ scores and impaired psychomotor (mental and motor) development. If properly controlled, often by increasing the amount of thyroid hormone, women with hypothyroidism can have healthy, unaffected babies.

Is hyperthyroidism dangerous during pregnancy?

Fortunately, hyperthyroidism during pregnancy is not common. Pregnant women with uncontrolled hyperthyroidism can develop high blood pressure. There is also an increased risk of miscarriage, premature birth, and having a baby with a low birth weight.

What is the side effect of propylthiouracil?

SIDE EFFECTS: Stomach upset, nausea, vomiting, mild rash/itching, or headache may occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly. Tell your doctor immediately if any of these rare but serious side effects occur: muscle/joint pain, change in the amount of urine.

How long should you take propylthiouracil?

Propylthiouracil should have some effect on your symptoms around three to four weeks after treatment starts. Your thyroid hormone levels should be stabilised within four to eight weeks. Continue to take the tablets for as long as you are advised to do so by your doctor.

Can hyperthyroidism cause birth defects?

Thyroid Disease Increases Birth Defects. Jan. 18, 2002 -- Women with thyroid disease are more likely to have a child with birth defects -- even if thyroid tests taken during pregnancy are normal. The risk for heart defects was seen in women with an underactive thyroid but not in women with an overactive thyroid.

Which is better PTU or methimazole?

Methimazole was superior overall, and lower doses seemed sufficient for patients with mild-to-moderate hyperthyroidism. Both methimazole and propylthiouracil (PTU) are used to treat hyperthyroidism. Rash was less common with lower-dose methimazole than with higher-dose methimazole or PTU.

What is propylthiouracil 50 mg used for?

Propylthiouracil (PTU) is an oral medication that is used to manage hyperthyroidism which is due to an overactive thyroid gland. It is an anti-thyroid drug that has a mechanism of action that is similar to methimazole (Tapazole). Graves' disease is the most common cause of hyperthyroidism.

Which medication is used to treat hyperthyroidism during the first trimester of pregnancy?

When and how to use antithyroid drugs during the first trimester of pregnancy. Antithyroid drugs, such as methimazole (MMI) and propylthiouracil (PTU) have been used to treat hyperthyroidism during pregnancy.

Can methimazole cause a miscarriage?

Hyperthyroidism in pregnancy is a rare, serious condition which can increase the risks of miscarriage, stillbirth, preterm birth, and intrauterine growth restriction. PTU is currently the favoured drug for use in pregnancy, as it is associated with fewer teratogenic effects (scalp lesions) than methimazole.

Is PTU safe?

PTU is indicated for hyperthyroidism due to Graves disease. These reports suggest an increased risk for liver toxicity with PTU compared with methimazole. Serious liver injury has been identified with methimazole in 5 cases (3 resulting in death).

Does PTU cause hair loss?

Severe or prolonged hypothyroidism or hyperthyroidism may result in hair loss. Hair loss is also a possible side effect of some antithyroid drugs, including methimazole and propylthiouracil (PTU). Doctors prescribe antithyroid drugs to treat an overactive thyroid.

What level of TSH is dangerous during pregnancy?

Many women with hypothyroidism on thyroid hormone replacement therapy have TSH levels above the desired 2.5 mU/L level in early pregnancy. Higher TSH levels (TSH levels > 4.5 mU/L) are associated with increased risk for miscarriage and should be avoided in early pregnancy.

Is Carbimazole contraindicated in pregnancy?

Although risks to the fetal thyroid from moderate doses of antithyroid drugs appear to be small, the use of higher doses of carbimazole in combination with thyroxine to prevent hypothyroidism should be avoided in pregnancy.

Can I take PTU while breastfeeding?

The amount of MMI excreted in milk is equal to MMI levels in serum. Due to its lower concentrations in milk, PTU was used for decades as the treatment of choice during breastfeeding. We recommend that PTU should not be prescribed for thyrotoxicosis during lactation.

How long does it take to get pregnant after starting levothyroxine?

When hypothyroidism is the reason for infertility, taking thyroid medication will enable most women to conceive, from as soon as six weeks after treatment, according to a study published in February 2015 in the IOSR Journal of Dental and Medical Sciences.

Can propylthiouracil cause hypothyroidism?

Propylthiouracil can cause hypothyroidism necessitating routine monitoring of TSH and free T4 levels with adjustments in dosing to maintain a euthyroid state.

Can you get pregnant while taking methimazole?

Methimazole is readily absorbed in the gastrointestinal tract, metabolized in the liver, and excreted in the urine. Patients should be advised that if they become pregnant or intend to become pregnant while taking an antithyroid drug, they should contact their physician immediately about their therapy.

Is it safe to take Tapazole while pregnant?

Because of the risk for congenital malformations associated with use of Tapazole in the first trimester of pregnancy, it may be appropriate to use other agents in pregnant women requiring treatment for hyperthyroidism. If Tapazole is used, the lowest possible dose to control the maternal disease should be given.