Context. The association between thyroid function and blood pressure is insufficiently studied.

Objective. To study the association between thyroid stimulating hormone (TSH) within the reference range and blood pressure.

Design and setting. Cross-sectional, population-based study.

Subjects. 30,728 individuals without previously known thyroid disease.

Main outcome measures. Mean systolic and diastolic blood pressure and pulse pressure, and odds ratio for hypertension (> 140/90 mmHg, 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/L), there was a linear increase in blood pressure with increasing TSH. The average increase in systolic blood pressure was 2.0 mmHg (95% CI, 1.4 to 2.6 mmHg) per mU/L increase in TSH among men, and 1.8 mmHg (95% CI, 1.4 to 2.3 mmHg) in women. The corresponding increase in diastolic blood pressure was 1.6 mmHg (95% CI, 1.2 to 2.0 mmHg) in men, and 1.1 mmHg (95% CI, 0.8 to 1.3 mmHg) in women. Comparing TSH of 3.0-3.5 mU/L (upper part of the reference) with TSH of 0.50-0.99 mU/L (lower part of the reference), the odds ratio for hypertension was 1.98 (95% CI, 1.56 to 2.53) in men, and 1.23 (95% CI, 1.04 to 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.