Pregnancy warning for medicines


Two anti-thyroid drugs have had their categories changed, with strong precautions issued for women of childbearing potential and pregnancy

Health professionals are advised that the pregnancy category for both propylthiouracil and carbimazole is being changed from category C to category D, after cases of congenital abnormalities were reported in the post-market setting following use of these medicines.

The medicines should not be prescribed for women of childbearing age unless – in the opinion of the doctor, the potential benefits outweigh the possible risks – according to a Medicines Safety Update from the TGA.

Propylthiouracil is an antithyroid drug indicated for the total treatment of hyperthyroidism or in the treatment of the thyrotoxic patient prior to surgery or radioactive iodine therapy, marketed in Australia under the brand name PTU.

Carbimazole is also an antithyroid drug indicated for hyperthyroidism. It is used as a definitive therapy in induction of a permanent remission in either primary or secondary thyrotoxicosis.

Carbimazole, which is also used in preparation for thyroidectomy before and after radioactive iodine treatment, is marketed in Australia under the brand name Neo-Mercazole.

Reviewing a safety signal relating to congenital abnormalities for propylthiouracil and carbimazole, the TGA found that the current Australian PI documents for both medicines contained sufficient safety information under ‘Section 4.6 Fertility, Pregnancy and Lactation’.

Additionally, the carbimazole PI contains additional information on women of childbearing potential and pregnancy in ‘Section 4.4 Special Warnings and Precautions for Use’.

However, the products were previously categorised under pregnancy category C, which is defined as ‘Drugs which, owing to their pharmacological effects, have caused or may be suspected of causing, harmful effects on the human foetus or neonate without causing malformations. These effects may be reversible’.

“As cases of congenital abnormalities have been reported in the post-market setting following use of these medicines, this category is no longer considered the correct pregnancy categorisation,” said the TGA.

“Category D is defined as ‘Drugs which have caused, are suspected to have caused or may be expected to cause, an increased incidence of human foetal malformations or irreversible damage’ and reflects the post-market experience with these medicines.”

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1 Comment

  1. Ron Batagol
    16/09/2021

    I’m not surprised that these medications are now listed as Category D in pregnancy. Both PropyIthiouracil and Carbimazole cross the placenta and can affect fetal development.

    Indeed, it has been well established that the preferred management of hyperthyroidism in pregnancy, under strict obstetric supervision, is to give Propylthiouracil in the 1st trimester, to minimize the potential for congenital defects of Carbimazole. Then, because of the potential for Propylthiouracil to sometimes cause severe liver damage, pregnant women are usually changed to Carbimazole after the first trimester, which is continued for the remainder of the pregnancy, again under strict obstetric supervision.

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