Thyroid function, as assessed by TSH, and future risk of venous thromboembolism: the Tromso study

Objective: The relationship between thyroid function and the risk of venous thromboembolism (VTE) has not been addressed in population-based cohorts. We investigated the association between TSH levels and the risk of VTE in a general adult population. Design: Population-based cohort study. Methods: TSH was measured in 11 962 subjects aged 25-89 years who participated in Tromso 4-6 starting in 1994-1995. Incident VTE events were recorded through 31st December 2010. Cox's regression models with TSH as a time-varying covariate were used to calculate hazard ratios (HRs) of VTE by TSH categories (low TSH: < 0.05 mU/l; moderately reduced TSH: 0.05-0.19 mU/l; normal TSH: 0.20-4.00 mU/l; moderately elevated TSH: 4.01-5.00 mU/l; and high TSH: > 5.00 mU/l) and within the normal range of TSH, modeling TSH as a continuous variable. Results: There were 289 VTEs during 8.2 years of median follow-up. Subjects with low(prevalence: 0.22%) and high (3.01%) TSH had slightly higher risk estimates for VTE than did subjects with normal TSH (multivariable HRs: 2.16, 95% CI 0.69-6.76 and 1.55, 95% CI 0.87-2.77 respectively), but the CIs were wide. Moreover, there was no association between TSH within the normal range and VTE (HR per 1 mU/l increase: 0.95, 95% CI 0.82-1.11). Conclusion: Serum levels of TSH within the normal range were not associated with a risk of VTE, whereas low and high TSH levels were rare and associated with a moderately higher risk of VTE. The present findings suggest that only a minor proportion of the VTE risk in the population can be attributed to thyroid dysfunction.