Low blood sugar may be caused by a primary disease, such as insulinoma and liver cancer. If the primary disease is treated well, the blood sugar can usually be restored. 1. Insulinoma: It is the most common insulin-secreting functional neuroendocrine tumor of pancreas, about 90% of which are benign tumors. The common clinical manifestation is fasting hypoglycemia. Surgical resection of insulinoma is the first choice of treatment. 2. Hepatocellular carcinoma: The main systemic manifestations caused by hepatocellular carcinoma are erythrocytosis and hypoglycemia; the secondary manifestations are hypercalcemia and hyperlipidemia. Its treatments include surgical resection, chemotherapy and so on. If there is a case of low blood sugar which cannot be replenished, one must go to the hospital for diagnosis and treatment in time.