Yes, THEREMINS is a Scrabble word!

Letter values in Scrabble US: T 1, H 4, E 1, R 1, E 1, M 3, I 1, N 1, S 1.

Is it a valid word?

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

an electronic musical instrument; melodies can be played by moving the right hand between two rods that serve as antennas to control pitch; the left hand controls phrasing