Why ThreadId changes when I set the max number of threads in a Threadpool to 1

I need a way to determine what threadId a process is running on.  Please see the following code.  I have a background worker that gets invoked when you click on button1 on a form.  If I set the max number of threads to 2, then I am expecting that the label on the form will only show 2 different threadids no matter how many times I rapidly click on the button... but that isn't happening.   Instead, I may get 3 or 4 different threadids.  

If I set the max number of threads to 1 and click on the button1 to launch background workers, then I expect that I should only see one threadid over and over gain.  Instead I get several different thread ids.

I'm not sure if my threadpool is not working or if I need to use a different way to get the threadid of the thread that is executing.

Can anyone tell me how to determine what threadid the current thread is running on.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Threading;
using System.Windows.Forms;

namespace Deleteme_Test_MultiThread
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private System.ComponentModel.BackgroundWorker worker;
        private string results = "";
        private int ireturn = 1;

       
        private void button1_Click(object sender, EventArgs e)
        {
            int iThreadMax = Convert.ToInt16(this.txtMaxThreads.Text);
            ThreadPool.SetMaxThreads(1, 1);

            worker = new System.ComponentModel.BackgroundWorker();
            worker.WorkerReportsProgress = true;
            worker.WorkerSupportsCancellation = false;
            worker.DoWork += new DoWorkEventHandler(this.DoWork);

            worker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(this.WorkCompleted);
            worker.ProgressChanged += new ProgressChangedEventHandler(this.ProgressChanged);

            //fire off worker
            worker.RunWorkerAsync();
        }
       


       private void ProgressChanged(object sender, ProgressChangedEventArgs e)
       {
           this.label1.Text = results;
       }


        private void DoWork(object sender, DoWorkEventArgs e)
        {
            worker = (System.ComponentModel.BackgroundWorker)sender;
            worker.ReportProgress(0);
            int ithread = System.AppDomain.GetCurrentThreadId();

            //int ithread = System.Threading.Thread.CurrentThread.ManagedThreadId;

            Thread.Sleep(2000);
            ireturn++;
            results = ithread.ToString() + " this is a test " + ireturn.ToString();
           
        }

        private void WorkCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            this.label1.Text = results;
        }
    }
}