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Background Thyroid dysfunction during pregnancy has emerged as an important yet under-recognized contributor to adverse maternal and fetal outcomes. Both gestational diabetes mellitus (GDM) and thyroid disease share overlapping metabolic and hormonal pathways that may synergistically amplify obstetric risk. In developing countries such as India, where the dual burden of iodine deficiency and gestational dysglycemia persists, understanding this association is essential for optimizing antenatal care. This study aimed to evaluate the prevalence, determinants, and perinatal consequences of thyroid dysfunction among women with GDM in the second trimester. Methods A hospital-based cross-sectional analytical study was conducted among 105 pregnant women aged 18-40 years diagnosed with GDM using the DIPSI single-step test (2-hour plasma glucose ≥140 mg/dL after 75 g glucose). Women with pre-existing thyroid or systemic illness were excluded. Serum thyroid-stimulating hormone (TSH), free thyroxine (FT4), and anti-thyroid peroxidase (anti-TPO) antibodies were measured using the electrochemiluminescence immunoassay method. Thyroid dysfunction was classified using ATA pregnancy-specific reference ranges. Maternal demographic, biochemical, and obstetric parameters were analyzed using SPSS version 26. Logistic regression identified predictors of thyroid dysfunction, with p < 0.05 considered significant. Results Thyroid dysfunction was detected in 11.4% of GDM women, comprising mainly subclinical hypothyroidism. Women with thyroid dysfunction were significantly older (30.2 ± 4.8 years) and had higher BMI (28.5 ± 4.2 kg/m²) compared to euthyroid women (p = 0.042 and p = 0.008, respectively). Family history of thyroid disorder (41.7% vs 12.9%; p = 0.012) and anti-TPO positivity (33.3% vs 5.4%; p = 0.003) were strongly associated. On multivariate analysis, anti-TPO positivity (aOR 6.78; p = 0.016) and familial thyroid history (aOR 4.12; p = 0.047) independently predicted dysfunction. Insulin therapy was required more often (50% vs 24.7%), indicating greater metabolic derangement. Adverse neonatal outcomes were higher - macrosomia (41.7% vs 19.4%; p = 0.048) and NICU admission (33.3% vs 12.9%; p = 0.045) - among women with thyroid dysfunction. Conclusion The coexistence of thyroid dysfunction and GDM in mid-pregnancy is clinically significant and frequently autoimmune in nature. Routine screening for thyroid function and anti-TPO antibodies in GDM women can facilitate early diagnosis, prevent perinatal complications, and improve maternal-fetal outcomes through timely intervention.