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Evaluation of the association between St. John's wort and elevated thyroid-stimulating hormone
1Centre for Evaluation of Medicines, Father Sean O'Sullivan Research Centre, and McMaster University, Hamilton, Ontario, Canada.
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.
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