Hey ,
You can use below code for the same.
<header text="xyz" id="Yourheader" />
<script>
$(function(){
$("#dropdownid").change(function(){
if($("#dropdownid option:selected").text() =="Your text")){
$("#Yourheader").hide();
}
else
{
$("#Yourheader").show();
}
});
});
</script>
When you change your dropdown that time this function will call and do that proper action.