Welcome to Ask The Doctor.
I understand your concern.
If you are unable to gain weight, you should be evaluated for organic causes behind it. It can however be just constitutional. But it is not uncommon to have some organic causes behind this problem, if there is any organic cause, it should be evaluated.
If you have any loose stools, pain abdomen, anemia then you would require evaluation for Celiac Disease.
In case you have fever, weight loss then you would require evaluation for intestinal tuberculosis.
I would advise you to get anti TTG Anti body test done to rule out Malabsorption syndrome like Celiac disease. You should also get a Thyroid profile done to rule out any issues like Hyperthyroidism that can cause weight loss. If you have any cough, expectoration then get a Chest X-ray done to rule out any evidence of Tuberculosis.
If no cause is found, this might be just constitutional and requires just a good diet and exercise.
Share my opinion with your doctor.
I hope it helps.
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