Clinicians should be cognizant of the close relationship that exists between two of the most common endocrine disorders, primary hypothyroidism and diabetes mellitus. This applies to patients with both type 1 and type 2 diabetes mellitus(T1DM and T2 DM respectively). However, the association is greater in T1 DM, probably because of the shared autoimmune predisposition. In patients with T2 DM, the relationship is somewhat weaker and the explanation less clear-cut. Factors such as dietary iodine deficiency, metformin-induced thyroid stimulating hormone suppression and poor glycemic control may all be implicated. Further translational research is required for greater clarification. Biochemical screening for abnormal thyroid function in individuals who have diabetes is warranted, particularly in females with T1 DM, and therapy with L-thyroxine appropriately instituted if hypothyroidism is confirmed. Clinicians should be cognizant of the close relationship that exists between two of the most common endocrine disorders, primary hypothyroidism and diabetes mellitus. This applies to patients with both type 1 and type 2 diabetes mellitus (T1DM and T2 DM respectively). However, the association is greater in T1 DM, probably because of the shared autoimmune predisposition. In patients with T2 DM, the relationship is somewhat weaker and the explanation less clear-cut. Factors such as dietary iodine deficiency, metformin-induced thyroid stimulating hormone suppression and poor glycemic Further translational research is required for clarification. Biochemical screening for abnormal thyroid function in individuals who have diabetes is warranted, particularly in females with T1 DM, and therapy with L-thyroxine is instituted if hypothyroidism is confirmed.