Insulin dose requirements may vary in patients with type 1 diabetes mellitus (T1DM) and coexisting thyroid dysfunction. A retrospective study published in Diabetes & Metabolic Syndrome: Clinical Research & Reviews evaluated the association between thyroid hormone levels and insulin dosing in patients with T1DM and chronic autoimmune thyroiditis (CAT).

The study included 30 patients with T1DM, CAT, and hypothyroidism treated with levothyroxine (LT4). Insulin delivery was managed using an insulin pump system, and glucose levels were assessed with flash glucose monitoring (FGM). Basal and bolus fast-acting insulin doses were analyzed in relation to serum thyroid-stimulating hormone (TSH), free thyroxine (FT4), and free triiodothyronine (FT3).

Higher TSH levels were associated with higher basal and bolus insulin doses, with correlation coefficients of 0.77 and 0.74 (p<0.001). FT4 and FT3 levels were associated with lower insulin requirements, with correlation coefficients ranging from −0.71 to −0.82 (p<0.001). These associations remained independent of glycated hemoglobin (HbA1c), duration of T1DM, and body mass index (BMI).

Overall, higher TSH and lower FT4 and FT3 levels were associated with greater insulin requirements in this population. The cross-sectional design and small sample size limit interpretation, and the findings are considered hypothesis-generating.