Ottawa, Ontario – June 5, 2020
As you may be aware, the only manufacturer of propylthiouracil (PTU) in Canada recently stopped manufacturing it. The SOGC together with The Canadian Society of Endocrinology and Metabolism, is working with Health Canada to advocate for our patient’s critical need for PTU in the first trimester of pregnancy, and to help find solutions to restore reliable supply of the medication as soon as possible.
What is the current practice?
Mild/asymptomatic hyperthyroidism is usually not treated in pregnancy. However, patients who do need treatment with thionamides are usually started on/switched to PTU in very early pregnancy or preconceptually and then switched (back) to methimazole after the first trimester. The rationale for this practice is:
- PTU has lower teratogenic potential for birth defects and hence safer during the first trimester, in particular between 6 and 10 weeks of gestation.
- Methimazole has a higher teratogenic potential (aplasia cutis, esophageal atresia with tracheoesophageal fistula, omphalocele, and omphalomesenteric duct anomaly, patent vitellointestinal duct, choanal atresia and absent/hypoplastic nipples have been associated with its use).
- PTU carries a higher risk for the mother, in particular the risk of liver failure. That is the reason for switching to methimazole after the first trimester.
- Both methimazole and PTU are subject to some concern regarding potential for liver injury manifesting in the neonatal period.
What can we do in the interim?
Treatment should be individualized. One option may be to not treat maternal hyperthyroidism, particularly between 6 and 10 weeks of gestation. This will depend on the severity of the hyperthyroidism. If PTU is unavailable, the decision to not treat versus treat with methimazole needs to be based on a discussion of risks and benefit with the patient. If methimazole does need to be used in the first trimester, administration of the lowest possible dose to maintain the mother in a state of mild hyperthyroidism may need to be considered.
The SOGC is investigating other strategies to secure supply for individual patients on an as-needed basis while permanent solutions are being developed. We will update the membership on this and other strategies as this situation evolves.