Background: Subclinical hypothyroidism (SCH) in pregnancy is associated with adverse pregnancy and perinatal outcomes. However, few studies have investigated the evolution of postpartum thyroid function in these women. This study aimed to determine the postpartum outcomes of SCH during pregnancy and the clinical and biochemical factors related to the evolution of long-term hypothyroidism. Methods: A total of 393 women diagnosed with SCH during pregnancy (defined as thyrotropin [TSH] >4.0 μIU/mL with normal free thyroxine levels according to the 2017 American Thyroid Association guidelines) were prospectively followed up after delivery. Among them, 216 underwent long-term follow-up [median (interquartile range) follow-up time: 11 (7-19) months] postpartum. The clinical and biochemical characteristics of the women with long-term postpartum hypothyroidism and euthyroidism were compared. Linear mixed model (LMM) was used to explore the risk factors for longitudinal changes of TSH, and logistic regression analysis was employed to identify the independent predictors of long-term postpartum hypothyroidism. Results: The probability of long-term hypothyroidism after delivery in SCH during pregnancy was 38.9%. Among the subjects with normal thyroid function 6-week postpartum, 28.2% developed hypothyroidism during long-term follow-up. The LMM showed that gestational age at the time of SCH diagnosis (estimate: -0.018, p = 0.004) and thyroid peroxidase antibodies (TPOAb) (estimate: 0.001, p = 0.020) were significantly associated with longitudinal changes of TSH. The logistic regression model showed that TPOAb positive both during pregnancy and six-week postpartum was a risk factor for long-term hypothyroidism after delivery (odds ratio = 4.686 [95% confidence interval 1.242 to 17.680], p = 0.023). Conclusions: More than one-third of patients with SCH during pregnancy had persistent hypothyroidism after delivery. We recommend that patients with TPOAb positive both during pregnancy and six-week postpartum undergo close follow-up to detect persistent hypothyroidism, especially before the next pregnancy.
Keywords: follow-up; postpartum; pregnancy; subclinical hypothyroidism; thyroid peroxidase antibody.