Plasma triiodothyronine concentrations in breast cancer.

Plasma triiodothyronine (T3), thyroxine (T4) and thyroid-stimulating hormone (TSH) were measured in 44 early breast cancer patients, 48 with advanced breast cancer, 20 with cancer of the colon, and 56 healthy women of similar age. The plasma T3 concentrations were reduced significantly in both early and advanced breast cancer, but only in those colonic cancer patients with metastatic disease. The mean plasma TSH levels were higher in the breast cancer patients than in the other groups, but the increase was significant statistically only in the advanced disease. There was a negative correlation between plasma TSH and T3 in early breast cancer, but not in advanced breast cancer. All cancer groups had normal plasma T4 levels. It is concluded that a proportion of breast cancer patients are mildly hypothyroid, as judged by elevated plasma TSH and reduced T3 levels. In advanced cancer, other "nonspecific" factors also act to reduce the plasma T3.