Does testicular cancer cause a decrease in testosterone?

Yes it does. I have testicular cancer. I was recently diagnosed with it. My doctor told me that while you still have the cancer in your body it decreases testosterone and causes things like breast tenderness and enlargement. Also you may become more emotional. But the treatment which is a nightmare in itself. Removing the affected testicle through surgery will restore your testosterone to normal levels. In certain cases men take medication to boost testosterone. Ask your doctor.