Background: There is evidence to suggest an association between hypothyroidism (overt and subclinical) and cardiovascular disease. It is unclear whether the set-point of the thyroid axis, within the normal range, may be related to vascular risk.
Methods: We have reanalysed the data from the original Whickham study to assess BP in the entire cohort and its relationship to serum TSH within the reference range (0.36.0 mIU/L). Multivariate stepwise linear regression analysis was used with systolic and diastolic BP as dependent variables and age, gender, smoking, BMI, total cholesterol, triglycerides and serum log-normalised TSH levels as independent variables. Participants taking antihypertensive or thyroid medications were excluded to leave 2,345 subjects for analysis.
Results: In individuals excluded due to antihypertensive and thyroxine treatment (n=293), mean TSH (S.D.) was 2.4 mIU/L (1.5), whereas in the group analysed the values were 2.1 mIU/L (1.4). Similarly, in the excluded group, the BP (systolic/diastolic) was 145/85 compared to 139/85 in the analysed group (significant difference only in the systolic BP).
There was a significant positive association between serum TSH and systolic BP, independent of other variables (Beta coefficient =0.04, t=2.6, P=0.009). All other variables, except total cholesterol levels, were also significantly associated with systolic BP in the final model (adjusted r2=0.38). Systolic BP analysed by ANOVA in groups divided by quintiles of serum TSH ranged from 135.4 mm Hg (lowest) to 140.1 mm Hg (highest), and the lowest group was significantly different from the upper two groups (5.4 and 4.6 mm Hg, P<0.05 for both). There was no significant relationship observed between TSH levels and diastolic BP.
Conclusion: Systolic BP is modestly but significantly associated with TSH levels, within the normal reference range. The set-point of the hypothalamic-pituitary-thyroid axis may have a subtle influence on blood pressure.