Hi Jeff,
THREADPOOL is definitely a problem of a heavy used system with lots of requests at the same time. To fight against it I would of course check the parallelism settings and the most expensive parallel queries. Another issue could be "blocking" but first of all I would fight the parallelism 🙂
Microsoft Certified Master: SQL Server 2008
MVP - Data Platform (2013 - ...)
my blog: http://www.sqlmaster.de (german only!)