Hormone levels may explain differences in risk for some of the most commonly diagnosed female cancers and could be responsible for as many as 15% of cancers in the UK1.
The lifestyle choices that determine hormone levels are not as straightforward as stopping smoking or eating healthily and are largely dictated by reproductive or genetic factors, over which individuals may have little or no control.This page presents information on cancer risk and hormones including oral contraceptives and hormone replacement therapy.
Table 5.1 shows a summary of what we know about hormones and cancer risk.
Studies assessing serum levels of testosterone and other male sex hormones in men and risk of prostate cancer suggest a more than two-fold increase in risk in men with the highest testosterone levels2.
Risk of ovarian, endometrial and breast cancers increases with earlier age at menarche and later menopause, and reduces with each full-term pregnancy (the first full-term pregnancy providing more protection than successive ones)3,4. There is a transient increase in risk of breast cancer with each full-term pregnancy after the age of 25, and this is most marked with a woman’s first pregnancy. Uniparous women with a first full-term pregnancy after the age of 30 have a higher breast cancer risk than childless women until their late 50s5.
Breast cancer risk is reduced by around 4% for each year a woman breastfeeds4. It has also been reported that the risk of endometrial cancer is reduced by as much as 50%6 and the risk of ovarian cancer by 40%7 in women who breastfeed. Today around 70% of mothers breastfeed initially, compared with only 50% in the 1970s, but less than half of these women breastfeed for more than four months and only a third for more than six months8.
A meta-analysis of European case-control studies indicates that use of OCs for five or more years results in a 50% reduction in risk of ovarian cancer which remains for more than 20 years9. Case-control studies have consistently reported a 40 to 60% reduction in risk of endometrial cancer, increasing with duration of use, among users of combination OCs10. There is a slight transient increase in risk of breast cancer while OCs are taken11,12.
Use of OCs for more than five years results in a duration-dependent increased risk for cervical cancer after adjustment for potential confounders including human papillomavirus (HPV) status (see infections section below), smoking and use of barrier contraceptives. Risk falls once use of OCs is stopped13.
Women taking oestrogen-only HRT are at an increased risk of endometrial and ovarian cancers14. The risk of uterine cancer is approximately doubled after five years, while the risk of ovarian cancer increases by about 25% for the same duration of use3,10. Combined oestrogen-progestagen HRT does not increase risk of ovarian cancer, and may even protect against endometrial cancer 3,10,14.
The UK ‘Million Women Study’ reported breast cancer RRs for current use of oestrogen-progestagen therapy or oestrogen only of 2 and 1.3, respectively. Risk increased with duration of use, with risk ratios of 2.3 for 10 or more years of combined HRT, compared with 1.5 for less than one year. There was a marginally increased risk in women who stopped HRT less than a year before entering the study, but no difference in risk between never users and women as a group who stopped therapy at some point in the previous five years15.
It is estimated that over the past decade, use of HRT by UK women aged 50-64 has resulted in an extra 20,000 breast cancers, oestrogen-progestagen therapy accounting for 15,000 of these15.