How to Shrink a Cotton T Shirt

Organic cotton is the same as cotton, just that it has been grown without the application of conventional chemicals. Its fiber structure remains the same, and as such, it will respond to the treatment in much the same way that a conventional cotton t-shirt will. However, and this is important, many conventional cotton t-shirts will have been treated with chemicals to reduce shrinkage; as such, an organic cotton t-shirt risks shrinking more and this possibility must be accounted for when seeking to shrink it, or you risk over-shrinking.

Separately, yes this is possible: For shrinking polyester, see How to Shrink Polyester and for shrinking cotton, refer to the steps above for a cotton t-shirt, or see How to Shrink Cotton Fabrics for other cotton garments. It is harder to shrink a polyester and cotton blend because the whole purpose of combining polyester with cotton is to prevent shrinkage and produce an easy-care garment. You could try subjecting a top made from both materials to high heat washing and drying but don't expect much shrinkage to occur.

You can try by using warm water and an addition such as baby shampoo, then attempting to reshape the item with gentle pulling, then washing in cold water to prevent repeated shrinkage. For some help with doing this, see How to Unshrink Clothes. Stretching t-shirt fabric can cause it to become over-stretched and floppy though, so do stretch it with care and follow the principle of "a little goes a long way."