Low testosterone levels in men with low-risk prostate cancer are an early warning sign that their cancer might be getting aggressive, new study has found.

Prostate cancer is the cancer that forms in the tissues of the prostate, which is a reproductive organ found in front of the rectum. According to National Cancer Institute, around 233,000 cases of the cancer will be diagnosed this year.

Low-risk prostate cancer is when the cancer is developing slowly and is not threatening the life of the patient. These patients are often put under active surveillance to detect any changes in cancer growth. Recent research has shown that being under surveillance is better for men with low-risk prostate cancer than undergoing treatment.

Doctors fear that treatment for these lower-grade cancers will lead to unneeded side-effects.

The new study from Pontificia Universidad Católica de Chile shows that testosterone levels could be used as an indicator of prostate cancer progression.

The study was based on 154 men with low-risk prostate cancer. Researchers followed these patients for around 38 months and found that those with low testosterone levels had higher risk of developing severe cancer.

"These results suggest low levels of testosterone are associated with more aggressive prostate cancer. This contradicts long-held beliefs that high testosterone is risky for prostate cancer, and low testosterone is protective," said Ignacio San Francisco, MD, of the Pontificia Universidad Católica de Chile, lead author of the study.

According to researchers, the study findings could help treat prostate cancer patients better.

"In borderline cases, the presence of low values of free testosterone may help determine whether it is more prudent to initiate treatment rather than continue observation," said Dr. San Francisco in a news release.

The study is published in the journal BJU International.

A related study had found that prostate cancer tests, which are used to determine the growth of the cancer, often underestimate the severity of the disease in around half the cases.

According to nhs.uk, there is considerable uncertainty associated with active surveillance and doctors must explain to patients that these tests often don't predict the severity of the cancer with accuracy.