There are many varieties of Thinsulate and so this is a generic answer. It is composed exclusively of fine fibers (65% Olefin/35% Polyester). It does a superior job of reflecting thermal radiation and not compressing, compared to high loft insulations like Polarguard or Primaloft. As a result, it is about 1.7 times warmer for thin applications under pressure, like gloves, boots, diving suits, and tight fitting fashion-wear.
It doesn’t compress well to allow stuffing into a small pack. It is much heavier and doesn’t provide any warmth advantage for applications (jackets and sleeping bags) requiring thicknesses above about 3/4”.