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Context Optimal thyroid status in pregnancy is essential in reducing the risk of adverse outcomes. The management of hyperthyroidism in women of reproductive age poses unique challenges and it is unclear how preconception treatment strategies impact on thyroid status in subsequent pregnancy. Methods We utilised the Clinical Practice Research Datalink (CPRD) database to evaluate all females aged 15-45 years, with a clinical diagnosis of hyperthyroidism and a subsequent pregnancy (January 2000 to December 2017). We compared thyroid status in pregnancy according to preconception treatment, namely, (1) antithyroid drugs up to or beyond pregnancy onset, (2) definitive treatment with thyroidectomy or radioiodine before pregnancy, and (3) no treatment at pregnancy onset. Results Our study cohort comprised 4712 pregnancies. TSH was measured in only 53.1% of pregnancies of which 28.1% showed suboptimal thyroid status (TSH >4.0 mU/L or TSH reference range). Pregnancies with prior definitive treatment were more likely to have suboptimal thyroid status compared to pregnancies starting during antithyroid drug treatment (OR = 4.72, 95%CI 3.50-6.36). A steady decline in the use of definitive treatment before pregnancy was observed from 2000-2017. A third (32.6%) of first trimester carbimazole-exposed pregnancies were switched to propylthiouracil while 6.0% of propylthiouracil-exposed pregnancies switched to carbimazole. Conclusion The management of women with hyperthyroidism who become pregnant is suboptimal, particularly in those with preconception definitive treatment, and needs urgent improvement. Better thyroid monitoring and prenatal counselling are needed to optimise thyroid status, reduce teratogenic drug exposure, and ultimately reduce the risk of adverse pregnancy outcomes.