Yes, DELTOIDS is a Scrabble word!

Letter values in Scrabble US: D 2, E 1, L 1, T 1, O 1, I 1, D 2, S 1.

Is it a valid word?

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

a large triangular muscle covering the shoulder joint and serving to abduct and flex and extend and rotate the arm

Also: deltoid muscle, musculus deltoideus.