I've noticed that cheap mini-sewing kits, which can often be purchased for one dollar or some similarly low amount, often do not specify the type of material used to make the thread. Presumably, the thread is made of some inexpensive synthetic material such as polyester. May one use such thread, or is there a concern it may contain shatnez? May one use the thread on wool or linen clothing?
According to Jerusalem Kosher News Shatnez guide - page 6 at the bottom normal sewing machine thread can not be linen, however button thread, upholstery thread, and thread that appears a bit thicker than normal sewing machine thread may be linen.
I draw the following conclusion from this. If it is not thicker than usual there is no concern.