Welcome to Ask The Doctor.
I understand your concern.
Following are the causes of weight loss or not able to gain weight at your age:
1. Malabsorption syndrome.
2. Inadequate diet
3. Intestinal Tuberculosis.
4. Normal constitution of the body.
I would advise following in such scenario:
1. To get anti TTG Antibody test done to rule out Malabsorption syndrome like Celiac disease.
2. Upper GI endoscopy
3. Ultrasound Abdomen.
4. Chest Xray
5. Thyroid profile
6. Hemogram with ESR.
Share my opinion with your doctor.
I hope it helps.
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