The 17-year-old boy is getting thinner and thinner may be caused by physiological factors, disease factors, hyperthyroidism, psychological factors and other reasons. 1. Physiological factors: 17-year-old boy is in the period of growth and development, energy demand, exercise, heavy learning pressure, if the daily intake of diet or energy is insufficient, may appear more and more thin. 2. Disease factors: there may be tumors, digestive diseases, some chronic tumors will consume the body’s nutrients, so it will cause wasting, digestive diseases will lead to poor appetite, nausea, vomiting phenomenon, so it will lead to 17-year-old boys more and more wasting. 3. Hyperthyroidism: Hyperthyroidism can cause hypermetabolism and sympathetic excitation, mainly because the thyroid gland synthesizes and releases too much thyroid hormone, clinical symptoms are manifested as palpitations, sweating, increased food intake, but weight loss and so on. 4. Psychological factors: there may be excessive mental stress, depression, anxiety and other bad emotions caused by wasting. It is recommended to go to the regular hospital for examination, the doctor will be based on clinical symptoms to give judgment, clear causes and timely symptomatic treatment.