Early pregnancy FT4 levels show non-linear association with gestational diabetes mellitus risk in 40,682 women.

This retrospective cohort study analyzed data from 40,682 pregnant women to assess the relationship between early pregnancy thyroid hormones and glucose metabolism. The population included women with gestational diabetes mellitus and normal glucose tolerance, comparing early pregnancy FT4 and TSH levels against normal thyroid function ranges and glucose tolerance subtypes. The primary outcome was GDM development, with secondary outcomes including OGTT glucose levels and macrosomia.

Analysis revealed a J-shaped association between FT4 and fasting/1-hour OGTT glucose levels. Specifically, FT4 levels below 15.4 pmol/L demonstrated strong protection against elevated glucose. However, low-normal FT4 levels ranging from 11.6 to 15.4 pmol/L were independently associated with an increased risk of GDM. Regarding macrosomia, low-normal FT4 was also associated with a higher risk. FT4 demonstrated strong capability for differentiation among GDM subtypes, whereas TSH showed limited effects with weaker associations regarding subtypes.

The study notes that FT4 is an independent risk factor for GDM. Safety data regarding adverse events, serious adverse events, discontinuations, and tolerability were not reported in the provided evidence. Key limitations regarding the study design and population characteristics were not explicitly detailed in the input data. Consequently, practice relevance and funding conflicts were not reported. The evidence is observational, and causal language is avoided. Clinicians should interpret these associations cautiously given the retrospective nature of the cohort and the lack of reported certainty notes or specific p-values.