You can be hypo and still be thin. That is totally possible. However if your test are normal than you are normal. There are a couple of different test they can do TSH, T4, and Free T4....all can show different results...Mine have done that at different times. You don't want to take thyroid replacements unless you are not normal with the blood tests. Many people feel fatigue have dry and brittle skin and hair. Those are signs of weather..stress..and many other things. If your thyroid is enlarged which mine is as well you can be normal and then swing low. You need to be checked every 3 months. I go from being on no meds to 25 mcg to 200 mcg..which is what I am on now.