- STUDY OBJECTIVE: To evaluate whether St. John's wort is associated with an elevation in thyroid-stimulating hormone (TSH) levels. DESIGN: Retrospective case-control study SETTING: Hamilton, Ontario; September 1998-May 1999. SUBJECTS: Thirty-seven subjects with elevated TSH levels and 37 age- (+/- 6 yrs) and gender-matched subjects with normal TSH levels who resided in the same region. MEASUREMENTS AND MAIN RESULTS: Exposure to St. John's wort during the 3-6 months before TSH measurement was assessed through a telephone interview. A total of 4 of 37 subjects with elevated TSH levels and 2 of 37 subjects with normal TSH levels had taken St. John's wort in that time period. The odds ratio for elevated TSH levels associated with taking St. John's wort was 2.12 (95% confidence interval [CI] 0.36-12.36). CONCLUSION: This study suggests a probable association between St. John's wort and elevated TSH levels. However, because of the wide CIs surrounding the point estimate (which crosses unity), further investigation, including a large, prospective cohort study, is warranted.