Abstract: CONTEXT: Associations between autoimmune diabetes and autoimmune thyroid disease are known but insufficiently characterized. Some evidence suggests that type 2 diabetes may also be associated with hypothyroidism. OBJECTIVE: The objective of the study was to investigate associations of autoimmune and type 2 diabetes with the prevalence of hypo- and hyperthyroidism. DESIGN AND SETTING: This was a cross-sectional, population-based study of adults in two surveys of the Nord-Trondelag Health (HUNT) Study. PARTICIPANTS: A total of 34 235 participants of HUNT2 (1995-1997) and 48 809 participants of HUNT3 (2006-2008) participated in the study. MAIN OUTCOME MEASURES: Prevalence of hypo- and hyperthyroidism was estimated, assessed by self-report, serum measurements, and linkage with the Norwegian Prescription Database. RESULTS: In HUNT2, autoimmune diabetes was associated with a higher age-adjusted prevalence of hypothyroidism among both women (prevalence ratio 1.79, 95% confidence interval [CI] 1.30-2.47) and men (prevalence ratio 2.71, 95% CI 1.76-4.19), compared with having no diabetes. For hyperthyroidism, the corresponding cumulative prevalence ratios were 2.12 (95% CI 1.36-3.32) in women and 2.54 (95% CI 1.24-5.18) in men with autoimmune diabetes. The age-adjusted excess prevalence of hypothyroidism ( approximately 6 percentage points) and the presence of thyroid peroxidase antibodies (8-10 percentage points) associated with autoimmune diabetes was similar in women and men. Type 2 diabetes was not associated with the prevalence of hypothyroidism. In HUNT3, associations were broadly similar to those in HUNT2. CONCLUSIONS: Autoimmune diabetes, but not type 2 diabetes, was strongly and gender neutrally associated with an increased prevalence of hypo- and hyperthyroidism and the presence of thyroid peroxidase antibodies. Increased surveillance for hypothyroidism appears not necessary in patients with type 2 diabetes.