She tells you she is trying to conceive. How does that change your management?

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Management needs careful adjustment because carbimazole carries a risk of teratogenicity, particularly in the first trimester — classically aplasia cutis, and less commonly choanal or oesophageal atresia. If pregnancy is confirmed or actively planned I would consider switching to propylthiouracil, which is preferred during the first trimester due to lower risk of congenital malformations. After the first trimester we often switch back to carbimazole because PTU carries a small risk of hepatotoxicity. The aim is to use the lowest effective dose to maintain maternal free T4 in the upper normal range, avoiding both maternal thyrotoxicosis and fetal hypothyroidism. I would involve a joint endocrine–obstetric team early, ensure close TFT monitoring and provide pre-conception counselling on biochemical stability before pregnancy.

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