Originally Posted by FBinNY

First of all, BTID thumb strain was common for mtn bikers using shifters that were unreasonably stiff. That problem is what led to the lighter action, more ergonomic levers we see today.

Odds are that your thumb will recover, unless there's an underlying issue like rheumatic arthritis.

Besides learning to relax and not use a death grip when riding, you might invest a few dollars in a grip exerciser of the type used by tennis players. If all else fails, the remedy might be to (gasp) switch to twist shifters. If this is too declassee for you, you might leave the original shifters in place as chamouflage.