Plasma prolactin and its relationship to risk factors in human breast cancer.

The prolactin concentration has been determined in plasma from ostensibly healthy women living on the Island of Guernsey. There were 102, 42, and 41 women who had a mother, sister or maternal aunt, respectively, with breast cancer. The remaining 184 women in this study claimed to have no known family history of breast cancer and were used as a control group. The increased risk of breast cancer due to family history was not associated with a raised mean prolactin level compared to the control group. However, in the luteal phase of the menstrual cycle, daughters of breast cancer patients had significantly raised levels of prolactin at 19.00 h. There were no abnormalities in the mean plasma prolactin levels for the above groups associated with differences in age at first child, age at menarche, interval between age at menarche and first child, and body weight of post-menopausal women. (All these factors have been reported to influence breast cancer risk). It is concluded that prolactin has no obvious function in the aetiology of breast cancer. If it is involved, the mechanism by which it acts must be subtle and concerned with the homeostatic control governing nycthemeral prolactin rhythms.