Pregnancy after Hemithyroidectomy: Increased Risk of Gestational Overt Hypothyroidism Despite Favorable Obstetric Outcomes: A Nationwide Study

Background: To assess the association between preconception hemithyroidectomy and the risk of gestational hypothyroidism and obstetric outcomes. Methods: This nationwide population-based retrospective cohort study compared euthyroid pregnant women with a history of hemithyroidectomy prior to conception with age- and body mass index–matched (81 ratio) pregnant women with intact thyroid glands. Data were retrieved from a comprehensive integrated healthcare database. The primary outcome was overt gestational hypothyroidism (elevated thyrotropin [TSH] with low free T4). Secondary outcomes included thyroid hormone replacement therapy, obstetric complications, and the interval from surgery to the onset of overt hypothyroidism during pregnancy. Results: A total of 521 pregnancies following hemithyroidectomy were compared with 4168 matched control pregnancies. Overt gestational hypothyroidism was significantly more frequent among women with prior hemithyroidectomy than among controls (22.5% vs. 1.9%, p < 0.001), and the risk was highest among those operated for thyroid cancer. Thyroid hormone replacement therapy during pregnancy was markedly higher in the hemithyroidectomy cohort (19% vs. 2.7%, p < 0.001). TSH levels exceeding 10 mIU/L occurred in 4.7% of exposed pregnancies compared with less than 0.1% of controls ( p < 0.001). The median interval from surgery to gestational diagnosis was 2.5 years (interquartile range, 1.0–5.4 years), with nearly two-thirds of affected women already meeting biochemical criteria at their first prenatal assessment. Notably, a profound surveillance gap was uncovered 29.5% of post-hemithyroidectomy women completely missed first-trimester TSH screening. Despite the higher burden of biochemical thyroid dysfunction, absolute rates of adverse obstetric outcomes did not differ significantly between cohorts. Conclusions: Preconception hemithyroidectomy is associated with a markedly increased risk of gestational hypothyroidism and a higher need for thyroid hormone replacement, particularly in women treated for thyroid cancer. These findings support closer biochemical monitoring during pregnancy, in line with current guidelines. Despite this increased risk, obstetric outcomes were similar to those of women with intact thyroid glands.