Muscle contractions or twitching within a muscle group powered by a single motor nerve fiber cause a twitch within the left thumb, according to MedlinePlus. Autoimmune disorders, exercise, stress, nutrient deficiencies, a lack of sleep and side effects of medication can cause twitching.
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. In addition, medical conditions that prompt metabolic disorders, such as kidney disease, uremia and low potassium, may cause twitching in various parts of the body, including the thumbs.
Nervous system disorders and serious diseases, such as Lou Gehrig's disease and spinal muscular atrophy, often promote twitching within the body. Twitching is also a symptom of weak muscles, also known as myopathy, and damage to nerves that lead to muscles, according to MedlinePlus. In severe cases, a seizure may cause twitching in the left thumb; however, the twitching and shaking is not just isolated to the hands when a seizure occurs, states Healthline.