The incidence of thyroid disease was examined prospectively in 97 consecutive patients with breast cancer (Group I) and was compared with that of 61 women with cystic breast disease (Group II) and that of 60 control women (Group III) with no breast problems. Thyroid enlargement was found in 47%, 49%, and 22% of those in Groups I, II, and III, respectively, and treatment with thyroid hormones was taken by 9.3% (I), 8.2% (II), and 5.0% (III) of the cases. The mean thyroid-stimulating hormone (TSH) concentration in those in Group I (5.4 +/- 2.2 microU/ml) was significantly higher than in Groups II (3.9 +/- 1.9, P less than 0.01) and III (4.0 +/- 1.8, P less than 0.001), whereas thyroid microsomal antibodies were detected in 13.4% (I), 9.1% (II), and 1.7% (III); mean triiodothyronine (T3) and thyroxin (T4) concentrations were similar in the three groups. When both TSH and T3 concentrations were taken into account, 24% and 17% of the patients, respectively, from Group I were no longer in the Group III range; the corresponding figures for Group II were 13% and 23%, respectively. These results indicate that breast cancer and thyroid disease are probably related, but not in a specific way, since benign mastopathy also seems to be associated with thyroid disturbances.

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