How to add custom field in shortcode?

Greeting all,

Am working on my project and trying to put the custom fields to the shortcode so I can use it in my posts.

add_shortcode('shortcodename', 'shortcodefunction');
function shortcodefunction() {
    return get_post_meta($post->ID, 'Description', true);
}

I want to return the value of the custom field “Description” but nothing shown. Wonder if there is anything wrong with the code above. And also wondering how I can add several custom fields to the shortcode, for example:

add_shortcode('shortcodename', 'shortcodefunction');
function shortcodefunction() {
    return '<div style="border:1px dotted #000; text-align:center; padding:10px;">
        <h4>'.get_post_meta($post->ID, 'Description', true).'</h4>
        <p><a href="'.get_post_meta($post->ID, 'URL', true).'">Stay up to date, and subscribe to my RSS feeds!</a></p>
        </div>';
}

Thank you very much!