Thyroid Dysfunction and Type 2 Diabetes Mellitus: Screening Strategies and Implications for Management

Diabetes mellitus (DM) and thyroid dysfunction (TD) often tend to coexist in patients. Both hypothyroidism and hyperthyroidism are more common in type 2 diabetes mellitus (T2DM) patients than in their nondiabetic counterparts. Current guidelines are neither clear nor specific about the frequency of thyroid function monitoring in T2DM patients. Circulating thyroid hormones affect several different organs and cells, have a major impact on glucose, lipid, and protein metabolism, and can worsen glycaemic control in T2DM. Hyperthyroidism and thyrotoxicosis can worsen subclinical DM and cause hyperglycaemia in T2DM patients, increasing the risk of diabetic complications. T2DM reduces thyroid-stimulating hormone levels and impairs the conversion of thyroxine (T4) to triiodothyronine (T3) in the peripheral tissues. Poorly managed T2DM can lead to insulin resistance and hyperinsulinaemia, which causes thyroid tissue proliferation and increases nodule formation and goitre size. In addition, while metformin can be beneficial in both T2DM and TD patients, other antidiabetics such as sulfonylureas, pioglitazone, and thiazolidinediones can negatively impact TD. Antithyroid drugs such as methimazole can impair glycaemic control in T2DM patients. Thyrovigilance in T2DM patients and diabetovigilance in TD patients may therefore be necessary to facilitate individualized care and management.