Thyroid function influences insulin requirements in patients with type 1 diabetes mellitus and chronic autoimmune thyroiditis: A cross-sectional study

OBJECTIVE: To evaluate the association between serum thyroid-stimulating hormone (TSH), free thyroxine (FT4), and free triiodothyronine (FT3) levels and basal and bolus fast-acting insulin doses in patients with type 1 diabetes mellitus (T1DM) and chronic autoimmune thyroiditis (CAT). PATIENTS AND METHODS: This retrospective study included 30 patients with T1DM, CAT, and hypothyroidism treated with levothyroxine (LT4), using the Omnipod DASH insulin pump system and flash glucose monitoring (FGM) (FreeStyle Libre 2). Insulin delivery parameters were analyzed in relation to thyroid function. RESULTS: Basal and bolus insulin doses were positively correlated with TSH (r = 0.77, p < 0.001 and r = 0.74 p < 0.001, respectively) and negatively correlated with FT4 (r = -0.75, p < 0.001 and r = -0.82, p < 0.001) and FT3 (r = -0.71, p < 0.001 and r = -0.82, p < 0.001), independently of HbA1c, T1DM duration, and body mass index. Patients with higher TSH and lower FT4 and/or FT3 levels required higher insulin doses. CONCLUSION: Reduced thyroid function in patients with T1DM and CAT is associated with increased insulin requirements, underscoring the importance of considering thyroid status in individualized insulin therapy. However, these findings should be interpreted with caution given the cross-sectional design and limited sample size, and should be regarded as hypothesis-generating.