Hi dear, Thanks for choosing Health care magic. If you only have muscle spasm and pain on right thumb with no other symptoms such as no spasm or pain on other fingers, no grip difficulty in your hands, no tingling etc. Then the reason for your condition could be due to dehydration, nutritional deficiency or a side effect to high blood pressure (if you have any) or blood pressure medications (such as Lisinopril). It could also be a result from a routine activity that you do on your right hand. Spams on thumbs could also happen due to other conditions such as, mechanical injury, fasciculation syndrome or Carpal tunnel syndrome. Carpal tunnel syndrome would be the closest mostly cause of your condition but this has other symptoms such as pain and spasms on fingers of that hand, or difficulty holding something. To rule out that you have Carpal tunnel syndrome, try to flex (stretch your wrist upward) for about 30 seconds to one minute and if you develop some sort of pain or numbness on fingers then its Carpal tunnel syndrome. I suggest that you stay hydrated so drink plenty of water at least 2 liters per day. If you see your symptoms progressing or not getting better after days of drinking sufficient amount of water, or you see you have Carpal tunnel syndrome then visit a Doctor so he can evaluate your condition thoroughly. The Doctor will ask you some questions such as if you had recent trauma on hand, what type of medication you take etc. The Doctor might suggest to do X-ray on hand and blood tests such as electrolyte profile see if there is any underlying condition... Hope this helped. Wish you all the best.