Driving me CRAZY :(

I have a very basic function that echoes the Category ID

<?php
function get_cat_id_now() {
	$category = get_the_category();
	$thecatid = $category[0]->cat_ID;
	echo $thecatid;
}
?>

and then, I want to use the include function together with the get_cat_id_now function.
Like this:
<?php include("/related/".get_cat_id_now().".html"); ?>

But it doesn’t work.
How do I add the get_cat_id_now() correctly into the include(/related/function_goes_here.html") ?