Turnbull was diagnosed with the cancer in November, and the disease has spread to his legs, hips, pelvis and ribs, Williams wrote in the Radio Times.
The former-TV, turned radio presenter is remaining positive, but wants the public to know about his diagnosis, so that they may consider being tested themselves.
Turnbull had prostate tests at both 40 and 50 years old, Williams said.
“Maybe if I’d got it earlier and stopped it at the prostate, I’d be in a much better state,” he told Williams.
He had noticed a change in his “pee pattern”, and had a number of aches and pains, but assumed it was just old age, he said.
These are the symptoms of prostate cancer to watch out for.
Prostate cancer doesn’t usually cause symptoms until it has grown large enough to put pressure on the urethra, according to the NHS.
The pressure usually results in problems related to urinating.
Needing to urinate more frequently – often during the night – and rushing to the toilet are signs of prostate cancer.
Other symptoms include difficulty in starting to urinate, straining or taking a long time to urinate, and having a weak urine flow.
If the cancer has spread, it can cause bone and back pain, a loss of appetite, pain in the testicles and unexplained weight loss.
Symptoms shouldn’t be ignored, but they don’t mean you definitely have prostate cancer, the NHS said.
It’s more likely to be caused by something else, such as prostate enlargement.
More than 40,000 new cases of prostate cancer are diagnosed every year in the UK.