Objective Overt hypothyroidism and hyperthyroidism are associated with hypertension, but it is uncertain whether the same is true of subclinical hypothyroidism and hyperthyroidism.Design, subjects and measurements Cross-sectional study of 2033 participants (aged 17-89 years) in the Busselton Thyroid Study who did not have a history of thyroid disease. Systolic blood pressure (SBP), diastolic blood pressure (DBP) and the prevalence of hypertension (defined as SBP 140 mmHg, DBP 90 mmHg or on treatment for hypertension) in subjects with thyroid dysfunction and euthyroid subjects were compared using linear regression models. Subjects with treated hypertension (N = 299) were excluded from analyses of SBP and DBP but included in analyses of hypertension prevalence.Results Mean SBP, DBP and the prevalence of hypertension did not differ significantly between subjects with subclinical hypothyroidism (N = 105) and euthyroid subjects (N = 1859), nor did they differ between subjects with serum TSH concentrations in the upper reference range (2.0-4.0 mU/l; N = 418) and those with TSH concentrations in the lower reference range (0.4-2.0 mU/l; N = 1441). The prevalence of hypertension was higher in subjects with subclinical hyperthyroidism than euthyroid subjects (prevalence odds ratio 2.8, 95% confidence interval 1.3-6.0 adjusted for age, age(2) and sex), but this was based on a small number of subjects with subclinical hyperthyroidism (N = 35).Conclusions Subclinical hypothyroidism is not associated with hypertension. The observed association between subclinical hyperthyroidism and hypertension requires confirmation in a larger sample.