CONTEXT: The association between thyroid function and blood pressure is insufficiently studied. OBJECTIVE: The objective of the investigation was to study the association between TSH within the reference range and blood pressure. DESIGN AND SETTING: This was a cross-sectional, population-based study. SUBJECTS: A total of 30,728 individuals without previously known thyroid disease were studied. MAIN OUTCOME MEASURES: The main outcome measures were mean systolic and diastolic blood pressure and pulse pressure and odds ratio for hypertension (>140/90 mm Hg or current or previous use of antihypertensive medication), according to categories of TSH. RESULTS: Within the reference range of TSH (0.50-3.5 mU/liter), there was a linear increase in blood pressure with increasing TSH. The average increase in systolic blood pressure was 2.0 mm Hg [95% confidence interval (CI) 1.4-2.6 mm Hg] per milliunit per liter increase in TSH among men, and 1.8 mm Hg (95% CI 1.4-2.3 mm Hg) in women. The corresponding increase in diastolic blood pressure was 1.6 mm Hg (95% CI 1.2-2.0 mm Hg) in men and 1.1 mm Hg (95% CI 0.8-1.3 mm Hg) in women. Comparing TSH of 3.0-3.5 mU/liter (upper part of the reference) with TSH of 0.50-0.99 mU/liter (lower part of the reference), the odds ratio for hypertension was 1.98 (95% CI 1.56-2.53) in men and 1.23 (95% CI 1.04-1.46) in women. CONCLUSION: Within the reference range of TSH, we found a linear positive association between TSH and systolic and diastolic blood pressure that may have long-term implications for cardiovascular health.