If you want to work with Unity and threads, you'll need to implement 2 features that are unfortunately absent from Unity: A thread pool and a main thread dispatcher.

using System;
using System.Collections.Generic;
using System.Threading;

public class UnityThreadPool : UnityEngine.MonoBehaviour
{       
    Deque<Action> mMainThreadWaitQueue;
    Deque<Action> mCurrentlyProcessing;
    static UnityThreadPool sInstance;
    Pool mPool;

    public void EnqueueOnMain(Action task) { lock (mMainThreadWaitQueue) mMainThreadWaitQueue.AddToBack(task); }
    public void EnqueueOnWorker(Action task) { mPool.QueueTask(task); }
    public static UnityThreadPool Instance {
        get {
            if (sInstance == null)
                sInstance = new UnityEngine.GameObject("ThreadPool").AddComponent<UnityThreadPool>();
            return sInstance;
        }
    }

    void Awake()
    {
        UnityEngine.Debug.Assert(sInstance == null,"Multiple ThreadPools detected.");
        DontDestroyOnLoad(transform.gameObject);
        mPool = new Pool(UnityEngine.SystemInfo.processorCount); 
        mMainThreadWaitQueue = new Deque<Action>(10);
        mCurrentlyProcessing = new Deque<Action>(10);
    }
    void Update()
    {
        lock (mMainThreadWaitQueue) {
            if (mMainThreadWaitQueue.Count == 0)
                return;
            UnityEngine.Debug.Assert(mCurrentlyProcessing.Count == 0);
            mCurrentlyProcessing.InsertRange(0,mMainThreadWaitQueue);
            mMainThreadWaitQueue.Clear();
        }           
        var watch = System.Diagnostics.Stopwatch.StartNew(); // this is the main thread, track time so we don't block it for too long
        while (mCurrentlyProcessing.Count !=  0){
            var action = mCurrentlyProcessing.RemoveFromFront();
            try { action(); } catch (System.Exception e) { UnityEngine.Debug.LogError("Unhandled Exception in MainThread task: " + e.ToString()); }
            if (watch.ElapsedMilliseconds > 10) {
                lock (mMainThreadWaitQueue) { mMainThreadWaitQueue.InsertRange(0,mCurrentlyProcessing); }
                mCurrentlyProcessing.Clear();
                break;
            }
        }
    }        
    void OnDestroy() { mPool.Dispose(); }
}

For an example implementation of Pool see: https://stackoverflow.com/a/436552/1612743