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A new study published in European Urology examines whether men with low concentrations of circulating free testosterone have a reduced risk of prostate cancer.

Researchers looked at blood samples collected between 1959 and 2004 from around 19,000 men aged 34 – 76 years. 6,900 of these men later developed prostate cancer.

They found that men with the lowest levels of testosterone had a 23% reduced risk of developing prostate cancer compared to all the other men. Interestingly, men with very low levels of testosterone who did get prostate cancer were more likely to develop an aggressive form of the disease.

Lead author, Ellie Watts, said “This is the first study which is large enough to look at whether unusually low testosterone levels influence the risk of developing prostate cancer. The findings are important because scientists know very little about the risk factors for developing the disease, including the role a man’s biology might play.”

Prostate cancer needs testosterone to grow. Nearly all prostate cancers that spread have overactive testosterone receptors, and the standard treatment for prostate cancer is hormone therapy which blocks or lowers the amount of testosterone in the body.

The study supports the hypothesis that stimulation of testosterone receptors in prostate cells is relatively constant. It appears that once testosterone rises above a low level, further increases in testosterone do not increase prostate growth. This could explain why only men with the lowest amount of the hormone, which falls below this point, have a reduced risk compared to men with higher levels of testosterone.

Professor Tim Key, co-author of the study said “This is an interesting biological finding that could help us understand how prostate cancer develops and progresses.”

Further research is needed to clarify whether the association of very low levels of circulating free testosterone and reduced risk of prostate cancer is causal or due to detection bias.