OBJECTIVES: Low maternal free thyroxine (fT4) levels have been associated with markers of insulin resistance and gestational diabetes. Our primary aim was to investigate whether maternal thyroid function was associated with subsequent insulin resistance in pregnant women with polyendocrine metabolic ovary syndrome (PMOS, previously known as PCOS). METHODS: In this post-hoc analysis of two randomized controlled trials, we used linear regression analyses to examine associations of thyroid stimulating hormone (TSH) and fT4 with parameters of metabolic profile (pre- and post-load glucose, fasting insulin and homeostatic model assessment of insulin resistance (HOMA-IR) in 284 pregnant women with PMOS who were randomized to metformin or placebo in early pregnancy. Blood samples were drawn at gestational weeks (gw) 5 to12, 19, 32 and 36. An oral glucose tolerance test was performed at inclusion, gw19 and 32. RESULTS: At baseline, the women had a mean body mass index (BMI) of 28.9 and HOMA-IR 3.2, and none had overt hypothyroidism. We found significant inverse associations between the change in TSH during pregnancy and post-load glucose at gw19, as well as fasting glucose at gw36, i.e. the smaller increase in maternal TSH, the higher subsequent maternal glucose levels. Furthermore, lower maternal fT4 levels were significantly associated with higher subsequent insulin/HOMA-IR levels, and the change in TSH was also inversely associated with HOMA-IR at gw32. All associations were independent of treatment with metformin or placebo. CONCLUSIONS: In this exploratory post-hoc analysis, a smaller maternal TSH increase and lower fT4 levels during pregnancy in overweight women with PMOS are associated with higher subsequent glucose and insulin/HOMA-IR levels later in pregnancy.