Transient Ischemic Attack (TIA): A TIA, often referred to as a "mini-stroke," is a temporary period of symptoms similar to those of a stroke. The intermittent nature of the numbness in the left hand could indicate a TIA, especially if it resolves on its own within 24 hours. This condition is a warning sign of a future stroke and requires immediate medical attention.

Migraine with Aura: Some migraines are preceded by neurological symptoms known as an aura, which can include numbness or tingling in parts of the body. If the numbness is accompanied by other migraine symptoms like headache, nausea, or visual disturbances, this could be a likely diagnosis.
Peripheral Neuropathy: Although less likely to cause intermittent numbness in a specific area like one hand without other symptoms, peripheral neuropathy can cause numbness, tingling, and pain, usually in the hands and feet. It's caused by damage to the peripheral nerves and can be due to various factors like diabetes, vitamin deficiencies, or alcohol abuse.
Cervical Spondylosis or Disc Disease: Compression or irritation of the nerves in the cervical spine can cause numbness or tingling in the hands. This condition is more likely if the numbness is accompanied by neck pain or if the symptoms worsen with certain neck movements.

Acute Ischemic Stroke: Although the question specifies intermittent symptoms, it's crucial not to miss an acute ischemic stroke, especially if the numbness is a new symptom and is severe or accompanied by other stroke symptoms like facial drooping, speech difficulties, or weakness in the arm or leg. Immediate medical attention is necessary for stroke treatment.
Subclavian Steal Syndrome: This is a rare condition where there's a significant stenosis or occlusion of the subclavian artery, leading to reversal of blood flow in the vertebral artery. It can cause symptoms like arm numbness or weakness, especially with exercise. Missing this diagnosis could lead to serious complications, including stroke.

Multiple Sclerosis (MS): MS is an autoimmune disease that affects the brain and spinal cord, leading to a wide range of neurological symptoms, including numbness or tingling in various parts of the body. The diagnosis of MS would require additional symptoms and diagnostic tests.
Neoplastic or Structural Lesions: Tumors or structural abnormalities in the brain or spinal cord can cause focal neurological symptoms like numbness. These would be rare causes of intermittent hand numbness without other symptoms but are important to consider in the differential diagnosis to ensure no serious condition is overlooked.