Gestational hypothyroidism and pregnancy outcomes following hemithyroidectomy: a nationwide matched cohort study

BACKGROUND: Hemithyroidectomy is increasingly performed in women of reproductive age, but its implications for thyroid function during pregnancy and obstetric outcomes remain unclear. OBJECTIVE: To assess gestational hypothyroidism and the risks of preterm birth and pregnancy loss among pregnancies in women with prior hemithyroidectomy versus matched pregnancies in women without thyroid surgery. METHODS: We conducted a retrospective matched cohort study using anonymized electronic medical records from Maccabi Healthcare Services, Israel, from 2002 to 2023. Pregnancies with at least one TSH measurement were included. Pregnancies after hemithyroidectomy were matched to pregnancies without thyroid surgery using exact matching on maternal age, gravidity, and pregnancy year, followed by propensity-score matching on sociodemographic and clinical covariates. Gestational hypothyroidism was defined as TSH >4.0 mIU/L or levothyroxine initiation during pregnancy and analyzed using generalized estimating equations. Pregnancy loss and preterm birth were analyzed using a weighted time-varying Cox model with gestational age as the time scale, incorporating thyroid status and levothyroxine treatment as time-varying covariates and a hemithyroidectomy-by-trimester interaction. RESULTS: The matched cohort included 463 pregnancies after hemithyroidectomy and 2,293 matched controls. Prior hemithyroidectomy was associated with higher odds of gestational hypothyroidism (adjusted odds ratio, 4.2; 95% CI, 2.9-6.3; p<0.01). In the time-varying outcome analysis, hemithyroidectomy was not associated with increased first-trimester pregnancy loss (hazard ratio [HR], 0.9; 95% CI, 0.7-1.1; p=0.29), but was associated with higher hazard of preterm birth or pregnancy loss during the second and third trimesters (HR, 1.6; 95% CI, 1.3-2.0; p<0.01). CONCLUSIONS: Prior hemithyroidectomy was associated with gestational hypothyroidism and modestly higher later-gestation hazard of preterm birth or pregnancy loss, supporting enhanced thyroid surveillance during pregnancy.