the_post_thumbnail (); in shortcode printed out the loop

Hi evryone,
I’m sorry for my english, I’ll try to explain.
Im working to my portfolio page with custom_post_type posted via shortcode. The portfolio is filtered by Isotope.js and the thumbnail is attached from a custom meta box. it’s working fine.
The problem is this, if i try to attach the post thumb with the_post_thumbnail(); and not with my meta box, the thumbs images are loaded before and out of the loop.

thi is the html resulted with the custom meta boxe:

<div class="portfolio">
....
....
<article class="post-222 portfolio type-portfolio status-publish hentry coll-5 fracassi" id="#">
     <div class="wrapper">
<a href="http://xxx.it" class="thumb">

<span class="x-preloader"></span>

<img alt="" src="http://www.xxx.it/test/wp_test/wp-content/uploads/2013/01/0d57b590560211e2bf5922000a1f8cdc_72-300x300.jpg">

</a>
<div class="info">
    <h3 class="title">Mi fracassi</h3>
         <span class="text"></span><div><h1>Test TEST</h1></div></div>
    </div>
       </article>

....
....
</div>

And this is the resoult with the the_post_thumbnail(); :

<strong><img alt="" src="http://www.xxx.it/test/wp_test/wp-content/uploads/2013/01/0d57b590560211e2bf5922000a1f8cdc_72-300x300.jpg"></strong>

<div class="portfolio">
....
....
<article class="post-222 portfolio type-portfolio status-publish hentry coll-5 fracassi" id="#">
     <div class="wrapper">
<a href="http://xxx.it" class="thumb">

<span class="x-preloader"></span>

</a>
<div class="info">
    <h3 class="title">Mi fracassi</h3>
         <span class="text"></span><div><h1>Test TEST</h1></div></div>
    </div>
       </article>

....
....
</div>

How can you see the <img> is coming out of the portfolio wrap…

here the shortcode:

/*-----------------------------------------------------------------------------------*/
/*	Portfolio Shortcode
/*-----------------------------------------------------------------------------------*/
function coll_portfolio($atts, $content = null)
{
    extract(shortcode_atts(array(
        'columns' => '3'
    ), $atts));

    $output = '';

    // start portfolio
    $output .= '<div class="portfolio">';
    // start filter
    $output .= '<ul class="filter">';
    $taxonomy = array ('port-cat', 'instagram-cat');
    $tax_terms = get_terms($taxonomy, 'orderby=none');
    // first item (all)
    $output .= '
<li><a href="#">' . __('All', 'framework') . '</a></li>
';
    foreach ($tax_terms as $tax_term) {
        $output .= '<li ><a href="#">slug . '">' . $tax_term->name . '</a>';
    }
    $output .= ''; // end filter

    // start items
    $output .= '<div class="items clearfix">';
    $loop = new WP_Query(array('post_type' => array ( 'portfolio', 'instagram'), 'posts_per_page' => -1));
    while ($loop->have_posts()) : $loop->the_post();
        global $post;
        // get thumbnail

        $cols = 'coll-' . $columns;
        // build
		if ('portfolio' == get_post_type ()) {
		$thumb = get_post_meta(get_the_ID(), 'thumbnail', true);
        $output .= "<article id='" . $post->post_name . "' class='" . join(" ", get_post_class($cols)) . "'>";
        $output .= '<div class="wrapper">';

		$output .= '<a href="' . get_permalink(get_the_ID()) . '">';
        $output .= '<span class="x-preloader"></span>';
        $output .= '<img src="' . $thumb . '" alt=""/>';
        $output .= '</a>';
		$output .= '<div class="info">';
        $output .= '<h3 class="title">' . get_the_title(get_the_ID()) . '</h3>';
        $output .= '<span class="text">' . get_the_excerpt() . '</span>';
		$output .= '<div><h1>porca paletta</h1></div>';
        $output .= '</div>';
        $output .= '</div>';
        $output .= '</article>';
           }
		elseif ('instagram' == get_post_type ()) {
		$thumbgram = the_post_thumbnail('thumbnail');
		 $output .= "<article id='" . $post->post_name . "' class='" . join(" ", get_post_class($cols)) . "'>";

        $output .= '<img src="' . $thumbgram . '" alt=""/>';
        $output .= '';
		$output .= '<div class="info">';
        $output .= '<h3 class="title">' . get_the_title(get_the_ID()) . '</h3>';
        $output .= '<span class="text">' . get_the_excerpt() . '</span>';
        $output .= '</div>';
        $output .= '</div>';
        $output .= '</article>';
			}  

    endwhile;
    wp_reset_postdata();
    $output .= '</div>'; // end items

    $output .= '</div>'; // end portfolio
    return $output;
}

add_shortcode('portfolio', 'coll_portfolio');

I have tried several solutions, but without success,
some of you maybe know something i miss…
If you need more code’s to see i can show.
Thank to every one!