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43 kg is a very low weight and your weight should increase. Weight is some what determined by genetics as well and if your father is also very thin, then your weight can be so explained. If not then you need to get evaluated. Hyperthyroidism is a common cause for low weight and thyroid profile should be checked.
Also, try eating calorie rich food with more fat and protein rather than carbohydrate. You may use gym for weight training as well with excess protein intake.
I would suggest you to meet a nutritionist to get a detailed diet plan.
Hope this helps,
Wish you a good health.
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