Twitching in the left thumb may also indicate an overdose of caffeine, stimulants or amphetamines or a drug side effect to estrogen, corticosteroids or diuretics, adds MedlinePlus. Diet is also a cause of left thumb twitching, especially when an individual fails to consume a balanced diet. When the body is unable to fight off ailments, such as an autoimmune disorder, twitching is often a side effect.