The small, black seed that Goldfinches are crazy about is not the same as the “thistle” that grows here in Texas. The thistle we all see growing in pastures and roadsides around here is Cirseum texanum. However, Goldfinches love the tiny seeds of the Guizotia abyssinica, which grows in semi-arid Africa and parts of India. This tiny seed is sometimes called Nyjer (nī-jĕr), which is a trademarked name. It's the only common birdseed that comes from overseas. How it came to be called "thistle" (incorrectly) is a mystery.
Since it has to be imported, the U.S.D.A. requires that it be heated (250 degrees for 15 minutes) at customs, to kill any stray weed seeds. This partially dries out the seed. So its “freshness” has a life of only 6 or 7 months. Even with all this government-mandated processing, Goldfinches love it, and eat it like candy instead of throwing it on the ground. But they won’t eat it if given last season’s, dried-out, Nyjer seed, which (unlike other birdseeds) doesn't remain viable for the following season.
It's expensive and, like toddlers with Halloween candy, is eaten voraciously by Goldfinches even though the energy it provides is minimal. So I like to put it out only as a special treat, mostly feeding the energy-packed black-oil Sunflower instead.