Yes, HYPOTONICITY is a Scrabble word!

Letter values in Scrabble US: H 4, Y 4, P 3, O 1, T 1, O 1, N 1, I 1, C 3, I 1, T 1, Y 4.

Is it a valid word?

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

(of a solution) the extent to which a solution has a lower osmotic pressure than some other
(of muscular tissue) the state of being hypotonic

Not playable in Scrabble US: HYPOTONUS.