How Can I Gain Weight While Having Digestive Issues And Frequent Bathroom Trips After Eating?

Hi doctor, iam 34 years old and my weight is just 46 KG iam unable to gain weight , iam very lean and thin what i eat after just few hours i have to go bathroom it is happing since long time at least 3-4 months it happens daily facing problem in digestion and facing large digestion problem . Iam having lean thyroid i take medicine of 25mg as per doctor but that is in control, how to gain weight