Should tamoxifen be stopped if it caused endometrium thickening? taking it for dcis, will it lead to endometrium cancer if continued?

Probably not: In a female in their 20's the discovery of dcis is unusual and raises possibility of the need for prevention of other breast disease with tamoxifen furthermore endometrium cancer is very unusual in a women on tamoxifen of this age and particularly if she still has periods most doctors if concerned by thickening will do a biopsy and resolve the concern.