What are trying to achieve? Are.you aware that to compute the md5 locally, you will need to download the full file ?
There is a special ftp extension that provide a server-side md5 to the client but it'd not generally supported. Some server also implement some similar proprietary functionality; you''ll have to check for your specific server.
You may want to have a look at the XCRC or XMD5 or XSHA1 commands, for example.