Thistle cannot be pulled out because when the root breaks it makes two plants. Every piece of root can make a new plant. The roots also go down 12-16 ft. Amazing!
Chemical control is the only way to kill thistle unless it’s possible to get out all the root. 
Since the thistle is under a hydrangea dip a paint brush in Roundup or other brush killer and paint the thistle leaves, take great care not to get any on your shrub. It may take several applications. Follow the directions on the label for killing thistle. If you don’t want to use a brush killer. Put down multiple layers of cardboard over the thistle seal all the edges so no light gets to the plant. Don’t cover all the soil around your shrub because it needs water and air for its roots. It will take a couple of years to get rid of it.