Please, i need help. How can I gain weight ?

I just can't bear being ill anymore. All I want is to be healthy and stop using stupid hormone replacement therapy.

I am 17 years old, missing period for 2 years.

I am 166cm and 48kg which gives me a bmi of 17.4 which is underweight. First of all – I've ALWAYS been slightly underweight, atleast as long as I can remember. My parents told me that even when i was born I had been thinner than a normal baby.

Second of all, I've NEVER starved myself. I didn't even skip meals, maybe a few times a month but that's it. I have always been eating 3 meals a day (normal meals, not just black coffee or some yogurt)

My parents said that my portions were small, so I increased them. Ive never had snacks but now im starting to add them. I don't do any sports, only walking quite a lot.

Also, a year ago I had depression episode which lasted 4 months, where I used food to cope and binged on a bunch of cookies and grilled cheese sandwiches every day. And despite that, I gained only 1 kg (48kg -> 49kg) 😮‍💨

I just don't understand ? Ive checked everything, my endocrinologist even thought that I had a damaged hypothalamus or tumor, cuz my thyroid is also underactive, but no – everything seems perfectly fine.

Worth noting – im vitamin D deficient, but im getting closer to reaching the normal levels