Yes, MASTHEADS is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, S 1, T 1, H 4, E 1, A 1, D 2, S 1.

Is it a valid word?

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

a listing printed in all issues of a newspaper or magazine (usually on the editorial page) that gives the name of the publication and the names of the editorial staff, etc.
the title of a newspaper or magazine; usually printed on the front page and on the editorial page
the head or top of a mast