Depends. Depending on when in your menstrual cycle the pelvic ultrasound was done, a 17 mm endometrium may or may not imply a serious issue, but it does warrant further investigation such as repeat ultrasound ( if one was done right before the period) or possible endometrial biopsy, if original ultrasound was done right after the cycle. Talk to your gynecologist.
Thick uterine lining. So you have had as us and the lining is thick.In order to determine if this is just a normal growth or cancer a sample of the lining will need to be taken, like a d&c. The pathologist will than look under the microscope and see if the cells are abnormal. Cancer cannot be diagnosed by x-ray the first time...You have to have tissue.
Maybe. If it is uniform, I am betting on a premalignant hyperplasia, but you want to have it sampled and sent to a pathologist. These can be tricky; see if you can get two of us to read it. I'm pleased that you did not say anything about invasion being detectable -- that's a good thing.