If you look at carefully, there are some typos on the submit button.
The
{$thread['threadlink']} already included the thread id. It won't be read by the action of the form (probably it is OK if you use friendly URL). So, you need to use an hidden input for the
tid if you don't use the friendly URL (you can replace the
{$thread['threadlink']} with
showthread.php).
The submit button has name. So it will be included on your URL. Consider to remove it.
BTW, is there any reason why you use form for that (is there any reason why you don't use usual link)?