Please help! What's wrong with my weight gain???!

Please help! What's wrong with my weight gain???

Can u tell me what's wrong with me? I'm totally desperate about this and i need your help, especially if u're a physician.

I'm 18, asian, male, 5'10' but only weight a crappy 110 pounds!
my body mass index is 14.4!
This condition of mine have existed since i was on 9th grade, and now i'm already a university student!
My friends are also said i have something wrong, cause my diet is above average person's diet ( my lunch and dinner portion is WAY ABOVE average person's), but i only defecates about once every 2 - 3 days.
I've did a thyroid lab check yesterday, which results as:
TSHs = 2.153 (Normal = 0.36 - 5.8)
T3 = 0.83 (Normal = 0.91 - 1.64)
T4 = 7.42 (Normal = 4.5 - 12)

so i guess it's not hyperthyroidism... so what is it with me?

It is hard to tell really! Your lab work is good, but just so you know, the TSH with a healthy thyroid on average is 1.5. Usually above 2 is a concern especially if you check 2 months later and 6 months later again and it is creeping upward...... Check for Hashimotos with TPO antibodies.. That is an autoimmune disorder that you might have... You are also a young male and you should have very fast metabolism - which is why you do not gain weight easily. I would try eating good quality food, with good fats in them (fish oio, avocados, coconut oil etc.) Some people are thin - by their genes so that would be normal too. I wish you good luck!