Ive been told by my doctor i need to gain weight. i have a very fast metabolism. how do i get myself at the correct weight without losing it in a min?

The best approach: really should be for your primary care physician to refer you to a nutrionist to crack some numbers based on what to eat for your caloric intake versus expenditures (exercise). And by the way, even if your thyroid looks OK on initial screening tests, a referral to an hormonal specialist aka endocrinologist would be completely appropriate if weight loss is excessive. Take care.