Prevalence and aetiology of thyrotoxicosis in patients with hyperemesis gravidarum presenting to a tertiary hospital in Cape Town, South Africa

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.

Automated summary

The incidence of thyrotoxicosis is high in women with hyperemesis gravidarum (HG), with the majority being hCG-mediated. About one-third of patients with Graves' disease are asymptomatic and rely on antibody testing for diagnosis. Free T4 levels decrease with clinical stabilization of HG, suggesting dehydration may influence measurements.