Yes, SYNTHESIZER is a Scrabble word!

Letter values in Scrabble US: S 1, Y 4, N 1, T 1, H 4, E 1, S 1, I 1, Z 10, E 1, R 1.

Is it a valid word?

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

an intellectual who synthesizes or uses synthetic methods
(music) an electronic instrument (usually played with a keyboard) that generates and modifies sounds electronically and can imitate a variety of other musical instruments

Not playable in Scrabble US: SYNTHESISER.