Neurologist told me that my thumb fasciculations can be caused only by ms, als or by anxiety, no third option.can this be true for specific twitches?

Maybe: Your symptoms could be due to anxiety, a finger muscle sprain, a finger or wrist nerve being pinched, as in carpal tunnel syndrome, or MS, but the latter is unlikely unless you have many other symptoms. Most likely you have a muscle sprain or nerve impingement, or possibly severe anxiety. That's what your PCP will look at.

Other things: Well, there are other things that can be associated with fasciculations, such as post strenuous exercise, trauma, a compressed nerve, and familial causes, as well as MS, ALS and anxiety. Not knowing your history, it is difficult to assess this well. Suggest you obtain another opinion or do a virtual consult with a neurologist on here on HealthTap.