More effect if also do the same on right thumb:
--- current state and acute conditions are generally reflected on left thumb
and chronic/deep-rooting on right thumb
--- women usually have left arm as leading for treatment where the problems
may be better reflected, and men - right arm.
--- the same treatment can be done on thumbs of feet; arms are Yang and legs are Yin
so one can further expand it.
So general energy balancing is using both thumbs for a session.
Basic energy points are also on feet but this is less comfortable for self-treatment.