The sustainability transition of the textile industry requires amongst other strategies circular approaches. Ecodesign guidelines and design for recycling are approaches that reduce resource consumption and textile waste. Garments are made of a large variety of different materials, from blended fibers to haberdashery items, colorants, and finishings, making it challenging to predict the composition of post-consumer textile waste. This mix of materials complicates recycling efforts, contributing to globally less than 1% of fiber-to-fiber recycling. This study investigates material compositions of one of the most popular and widespread garments: T-shirts. While about half of our sample contains cotton only, the other items contain two or more fibers, revealing huge variations in fiber blends, including varying degrees of elastane contents, which are not linked to functional requirements. These blends, especially the varying levels of elastane, increase costs and efforts for recycling, making fiber-to-fiber recycling less attractive and more expensive than new fiber production. They also contribute to avoidable microfiber pollution. Accordingly, this study underlines the requirements for providing detailed ecodesign guidelines and applying the extended producer responsibility to incorporate environmental lifecycle costs, to help shift the industry towards a circular economy.