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Thyroid Supplements and Breast Cancer

Bruce Armstrong, MB, PhD, MRACP; Hershel Jick, MD
JAMA. 1976;236(24):2744. doi:10.1001/jama.1976.03270250013007.
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To the Editor.—  The data reported by Drs Kapdi and Wolfe does not distinguish between the use of thyroid supplements and the conditions for which the supplements were given as possible risk factors for breast cancer.A further analysis of these data is shown in the Table.1,2 There is only a modest increase in the risk of breast cancer in women taking thyroid supplements (relative risk, 1.59 overall), and the trend toward increasing risk with increasing duration of therapy is not consistent over the three duration levels.In a study of drug use at the time of diagnosis of cancer,3 7 of 708 patients with breast cancer and 16 of 1,429 patients with other cancers were taking thyroid supplements. With the other cancer patients considered as controls, the relative risk of breast cancer in users of thyroid supplements was estimated to be 0.96, with 95% confidence limits of


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