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Graves disease treated with carbimazole before radioiodine — DFSRH MCQ

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EasyThyroid DiseaseGraves disease treated with carbimazole before radioiodineDFSRH

A 30-year-old woman with Graves disease attends for contraceptive advice. She is biochemically euthyroid while taking carbimazole 20 mg daily and is scheduled to receive radioiodine in 3 weeks. She currently relies on fertility-awareness methods but wishes to conceive approximately 4 months after radioiodine. She has had no vaginal intercourse for 5 weeks, and a high-sensitivity urine pregnancy test today is negative. She declines an intrauterine method and implant but is willing to use an effective user-dependent hormonal method, for which she has no contraindication. Which is the most appropriate contraceptive and preconception plan?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: D — Start effective contraception now, continue it for at least 6 months after radioiodine, then arrange preconception thyroid review

Explanation lettering: E = shown as A · D = shown as B · A = shown as C · C = shown as D · B = shown as E

Carbimazole exposure and the planned radioiodine treatment create two consecutive reasons to avoid pregnancy. The MHRA advises effective contraception throughout carbimazole treatment because its active metabolite crosses the placenta and is associated with congenital malformations, particularly with first-trimester and higher-dose exposure. Fertility-awareness methods are not classified as effective contraception in the context of a medicine with teratogenic potential. As pregnancy is reasonably excluded and she accepts a suitable hormonal method, this should be initiated now. Pregnancy should then be deferred for at least 6 months after radioiodine, not merely until carbimazole is stopped. Preconception review is required to confirm stable euthyroidism because radioiodine may subsequently cause hypothyroidism requiring levothyroxine titration. A leaves her inadequately protected during current carbimazole exposure. B ends contraception before the separate post-radioiodine restriction. D delays effective contraception despite current carbimazole use and omits assessment of post-treatment thyroid status. E is incorrect because changing to propylthiouracil does not make fertility awareness sufficiently effective or shorten the post-radioiodine interval; antithyroid treatment changes should also be directed by endocrinology rather than used to circumvent contraception advice.

Reference: Carbimazole: increased risk of congenital malformations; strengthened advice on contraception (18 February 2019) — https://www.gov.uk/drug-safety-update/carbimazole-increased-risk-of-congenital-malformations-strengthened-advice-on-contraception Medicines with teratogenic potential: what is effective contraception and how often is pregnancy testing needed? (21 March 2019; minor table updates in 2024) — https://www.gov.uk/drug-safety-update/medicines-with-teratogenic-potential-what-is-effective-contraception-and-how-often-is-pregnancy-testing-needed North West Thyroid Disease in Pregnancy Guideline (19 January 2026) — https://www.england.nhs.uk/north-west/wp-content/uploads/sites/48/2026/04/MMN-Thyroid-Disease-in-Pregnancy-Guideline-003.pdf