OBJECTIVE - To determine the relationship between subclinical hypothyroidism (SCH) and the prevalence of diabetic retinopathy in type 2 diabetic patients. RESEARCH DESIGN AND METHODS - A total of 1,170 type 2 diabetic patients were screened for thyroid function. There were 127 type 2 diabetic patients with SCH and 200 randomly selected euthyroid type 2 diabetic patients selected. Those with more severe than moderate nonproliferative diabetic retinopathy were classified as having sight-threatening diabetic retinopathy (STDR). RESULTS - The trend for severe retinopathy was significantly higher in the SCH group than in the euthyroid group (chi(2) = 20.43, P = 0.000). SCH was associated with greater prevalence of diabetic retinopathy, especially STDR [odds ratio (95% CI): 4.15 (2.17-7.96), P = 0.0001 after an adjustment for age, sex, duration of diabetes, A1C, BMI, hypertension, and LDL cholesterol. Even euthyroid patients with thyroid-stimulating hormone levels between 2.0 and <4.0 mu IU/ml had a higher rate of STDR then those between 0.4 and <2.0 mu IU/ml (P = 0.008). CONCLUSIONS - Type 2 diabetic patients with SCH are associated with an increased risk of STDR.