criacao de threads

procedure TForm1.Button1Click(Sender: TObject);
var
  i: Integer;
begin
  for i := 0 to Listbox4.Items.Count-1 do
    CriaThreads(ListBox4.Items[i]);
end;

procedure CriaThreads(Url: String);
var Thd: TThreadHttp;
begin
  Thd := TThreadHttp.Create(True);
  Thd.FreeOnTerminate := True;
  Thd.Url := Url;
  Thd.Resume;
  end;

procedure TThreadHttp.Execute;
begin
inherited;
if terminated then
exit
else
begin
  Http := TidHttp.Create(nil);
  try
  Str := HTTP.Get(url);
  except
  end;
  Synchronize(ShowPage);
  end;

end;