Yes, HYPOSTASIS is a Scrabble word!

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

Is it a valid word?

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

the suppression of a gene by the effect of an unrelated gene
the accumulation of blood in an organ
any of the three persons of the Godhead constituting the Trinity especially the person of Christ in which divine and human natures are united
(metaphysics) essential nature or underlying reality

Also: hypostasis of christ.