Yes, HYPOTHESES is a Scrabble word!

Letter values in Scrabble US: H 4, Y 4, P 3, O 1, T 1, H 4, E 1, S 1, E 1, S 1.

Is it a valid word?

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

a proposal intended to explain certain facts or observations
a tentative insight into the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena
a message expressing an opinion based on incomplete evidence

Not playable in Scrabble US: SURMISAL.