TNewThread = class(TThread)
  private
    { Private declarations }
  protected
    procedure Execute; override;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TNewThread.Execute;
begin
  Sleep(5000);
end;

procedure TForm1.btn1Click(Sender: TObject);
var
  NewThread: TNewThread;
begin
  NewThread:=TNewThread.Create(true);
  NewThread.FreeOnTerminate:=true;
  NewThread.Priority:=tpLower;
  NewThread.Resume;
end;