Yes, THISTLE is a Scrabble word!

Letter values in Scrabble US: T 1, H 4, I 1, S 1, T 1, L 1, E 1.

Is it a valid word?

THISTLE is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

any of numerous plants of the family Compositae and especially of the genera Carduus and Cirsium and Onopordum having prickly-edged leaves