Lower free thyroid hormone levels are associated with high blood glucose and insulin resistance; these normalize with metabolic improvement of type 2 diabetes

Abstract Objective This study aimed to explore the relationship of thyroid function and glucose metabolism and to investigate the changes in thyroid function after National Metabolic Management Center (MMC) management in type 2 diabetic mellitus (T2DM). Methods A total of 2000 euthyroid participants from MMC in Shanghai General Hospital and a community physical examination were recruited. They were categorized into four groups: normal glucose tolerance (NGT, n = 132), prediabetes (N = 147), DM well‐controlled (T2DM with glycosylated hemoglobin [HbA1c] ≤ 7%, n = 505), and DM uncontrolled (T2DM with HbA1c > 7%, n = 1216). The parameters were compared among the groups and their changes before and after MMC follow‐up in DM uncontrolled group were observed. Results Free triiodothyronine (FT3) and free thyroxine (FT4) levels varied significantly among groups. FT3 and FT4 levels negatively correlated with HbA1c level ( P < 0.05), and positively correlated with the estimated glucose disposal rate (eGDR) ( P < 0.05). Following MMC management, blood glucose and insulin resistance in the DM uncontrolled group were dramatically improved ( P < 0.001). Meanwhile, FT3 was elevated compared to the baseline (from 4.51 ± 0.78 pmol/L to 4.68 ± 0.87 pmol/L, P < 0.05), and reverse triiodothyronine (r‐T3) decreased from 1.03 ± 0.24 nmol/L to 0.92 ± 0.25 nmol/L ( P < 0.001). Thyroid‐stimulating hormone significantly decreased as well ( P < 0.001). Conclusions Decreased free thyroid hormone levels in normal range were associated with high glucose and insulin resistance. After MMC management, improvement of blood glucose and insulin resistance were accompanied by the restoration of low‐normal thyroid function.