Is there any association between the lower free thyroxine levels and increased risk for gestational diabetes? The answer is yes. Pregnant women with lower free thyroxine (FT4) levels or isolated hypothyroxinaemia face a higher risk of developing gestational diabetes, according to a large-scale meta-analysis published in the Lancet Diabetes & Endocrinology that included 25 prospective cohort studies.

Among 45,900 participants with available thyroid antibody data, isolated hypothyroxinaemia was linked to a significantly higher rate of gestational diabetes compared with euthyroidism (6.5% vs 3.5%; adjusted odds ratio [aOR] 1.52; p=0.0017).

Lower FT4 concentrations showed a non-linear association with increased gestational diabetes risk (p<0.0001). Higher free triiodothyronine (FT3) levels (aOR 1.18; p<0.0001) and higher FT3-to-FT4 ratios were also associated with elevated risk.

No significant links were found between gestational diabetes and subclinical hypothyroidism, thyroid antibodies, or most other thyroid function abnormalities.