Higher Thyroid fT3-to-fT4 Ratio Is Associated with Gestational Diabetes Mellitus and Adverse Pregnancy Outcomes

Aim: To determine the association between thyroid function and the risk of developing gestational diabetes mellitus (GDM) and adverse pregnancy outcomes. Methods: This case–control study was a sub-analysis of the BEDIP-N study, in which 199 GDM women were matched for age and body mass index with 398 controls. Thyroid-stimulating hormone (TSH), free thyroxine (fT4), free triiodothyronine (fT3), and thyroid peroxidase (TPO) antibodies were measured at 6–14 weeks and 26–28 weeks during pregnancy. TSH and fT4 were also measured in early postpartum in GDM women. Results: The fT3-to-fT4 ratio at 26–28 weeks was positively associated with GDM risk with an adjusted odds ratio (aOR for smoking, education, parity, ethnicity, gestational weight gain, and (family) history of diabetes or GDM) of 2.12 (95% CI 1.07; 4.23), comparing the highest with the lowest tertile. Higher fT3 levels and a higher fT3-to-fT4 ratio were associated with a less favorable metabolic profile with higher BMI and more insulin resistance during pregnancy and postpartum. Women in the upper fT3 tertile and the upper fT3-to-fT4 ratio had a higher rate of preeclampsia [4.6% (10) vs. 1.0% (2), p = 0.040, and 4.4% (9) vs. 0.5% (1), p = 0.020], gestational hypertension [8.3% (18) vs. 3.1% (6), p = 0.034 and 8.9% (18) vs. 2.0% (4), p = 0.003], and caesarean sections [29.4% (63) vs. 16.1% (31), p = 0.002 and 32.2% (65) vs. 12.7% (25), p < 0.001]. Conclusion: A higher fT3-to-fT4 ratio late into pregnancy was associated with GDM, adverse pregnancy outcomes, and an adverse metabolic profile in early postpartum.