TOC in custom WP_Query

I have a custom WP_Query that shows 1 post in a category page and I would like to show the TOC for this post with a shortcode.

$args = array(
   'post_type' => 'post', 
   'posts_per_page' => 1, 
   'p' => $ID_articolo,
);								
$the_query = new WP_Query( $args );

I’ve followed the help section without success.
Any suggestion?

Thank you.