Association Between Thyroid-Stimulating Hormone and Estimated Glomerular Filtration Rate in the Third Trimester of Pregnancy: A Retrospective Cross-Sectional Study in Euthyroid Women

<i>Background and Objectives</i>: This study investigated the relationship between thyroid function and renal parameters during the third trimester of pregnancy in euthyroid women, a physiological interaction that remains poorly characterized. <i>Materials and Methods</i>: In this retrospective, single-center cross-sectional study, 820 euthyroid pregnant women (≥28 weeks of gestation) were evaluated. Thyroid-stimulating hormone (TSH), free triiodothyronine (fT3), free thyroxine (fT4), serum creatinine, and estimated glomerular filtration rate (eGFR) were analyzed using tertile-based comparisons, correlation tests, and linear regression analysis. <i>Results</i>: Higher TSH levels were associated with slightly higher serum creatinine (<i>p</i> = 0.011) and a weak negative correlation with eGFR (r = -0.079, <i>p</i> = 0.023). Conversely, fT3 levels were positively correlated with eGFR (r = 0.106, <i>p</i> = 0.002) and inversely correlated with creatinine (r = -0.074, <i>p</i> = 0.035), while fT4 showed weaker associations. Regression analysis confirmed that fT3 (β = 0.099, <i>p</i> = 0.005) and fT4 (β = 0.083, <i>p</i> = 0.019) were independent positive predictors of eGFR. <i>Conclusions</i>: The correlations observed were statistically significant but clinically modest. Regression analysis confirmed that FT3 and FT4 were independent positive predictors of GFR, suggesting that subtle variations in thyroid activity may reflect physiological rather than pathological renal adaptations in late pregnancy. Monitoring TSH and fT3 may enhance understanding of maternal endocrine and renal interplay, though the clinical utility of such associations remains limited and warrants confirmation in prospective studies.