Hi B.A.

How would you describe your swing? Fast take away? Quick tempo?  A lot of lag at the top? Huge divots? Is your thumb extended down the grip or more tucked in next to your index finger?

Try some of the following:

Slow down the take away to where you maintain control of the club to where you could stop it at any moment. It should help prevent the weight of the club yanking your wrists back at the top of your back swing. Try maintaining a 90 angel between your forearm and club shaft. You thumb might be catching most of that load in a faster transition. Having your thumb more tucked in should help get the handle in your fingers instead of your palm for better wrist hinge and relieve the pressure on your thumb.

Source: had a similar problem years ago when people used to emphasize huge lag. 

Spud

Thanks Spud and Dave!

"How would you describe your swing? Fast take away? Quick tempo?  A lot of lag at the top?..."

Uhh, yes, exactly. No big divots though, I make great contact. My grip could be a bit off so I'm trying to make some adjustments for that and I'll work on a slower take-away as that is on my list of needed changes anyway for an ideal tempo (mine is way too quick). 

Thanks!

I experience the same pain as well as tingling in my left thumb joint.  I recently changed my hand placement in gripping the club and have been working to loosen my gripping the club to alleviate the resulting pain and loss of feeling.  I hope it helps!