请教：如何让线程多次执行

我在一个按钮事件中执行一个线程，第一次可以正常运行。运行完毕后再次点击按钮，程序没有反应，怎样可以让线程多次执行呢？
 Private th As System.Threading.Thread
 Private Sub btnTest_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnRecord.Click
        If btnTest.Text = "测试" Then
           btnTest.Text = "停止"
           th = New System.Threading.Thread(AddressOf mydelegate)
            th.Start()
        Else
            btnRecord.Text = "测试"
            th.Abort()
        End if
End Sub