Background: The association between hyperemesis gravidarum (HG) and abnormal thyroid function is well known.Aims: The prevalence, aetiology and course of thyrotoxicosis in women with hyperemesis gravidarum (HG) were studied.Methods: Women admitted for HG, who underwent thyroid function evaluation between 1 August 2016 and 30 April 2019, were studied. Laboratory data included baseline human chorionic gonadotropin (hCG) and baseline (t1), discharge (t2) and follow-up (t3) thyroid function tests (thyroid stimulating hormone [TSH] and free thyroxin [fT4]). Available TSH receptor antibody status was assessed.Results: Eighty-two patients were included. The incidence of thyrotoxicosis was 49% based on local laboratory TSH range and 48% if trimester-specific ranges used. In the majority of normal pregnancies, thyrotoxicosis was hCG-mediated (72.5%), 15% were confirmed to have Graves’ disease and 12% had a molar pregnancy. Very high fT4 levels (> 40 pmol/l) at baseline [t1] were documented in 24% of women with hCG-mediated thyrotoxicosis. Clinical features were absent in a third of women with Graves’ disease and the diagnosis was reliant on positive antibody status. Free T4 values declined from (t1) to later in gestation (t3) (p < 0.001).Conclusion: The incidence of thyrotoxicosis in women with HG is high. Free-T4 values decrease with clinical stabilisation of HG, suggesting a contribution of dehydration to the large variation in baseline fT4 measurements. Testing for TSH-receptor antibodies should be considered in women with TSH < 0.01 pmol/l and persistent fT4 elevation on follow-up. Final review of thyroid function should be performed after 15 weeks’ gestation
