FT3

This study explored the association between free T3 (fT3), the fT3/fT4 ratio, and gestational diabetes mellitus (GDM) in a case-control analysis of 107 GDM cases and 214 controls within a diverse pregnancy cohort. Thyroid markers were assessed across pregnancy, including the first (10–14 weeks) and second (15–26 weeks) trimesters. Results showed that both higher fT3 levels and the fT3/fT4 ratio were significantly associated with an increased GDM risk. Adjusted odds ratios (OR) comparing the highest to the lowest quartiles for fT3 were 4.25 (95% CI 1.67, 10.80) in the first trimester and 3.89 (95% CI 1.50, 10.10) in the second trimester. For the fT3/fT4 ratio, the corresponding ORs were 8.63 (95% CI 2.87, 26.00) and 13.60 (95% CI 3.97, 46.30). No significant associations were found for TSH or fT4. These findings suggest that elevated fT3 levels and the fT3/fT4 ratio early in pregnancy may serve as indicators of increased GDM risk, possibly due to enhanced fT4 to fT3 conversion or increased de novo synthesis.