When testing your thryoid levels which end means what?
If it on the high side you gain weight and the low side you can't gain weight.

If you have hyperthyroidism, which would be on the high side, one of the symptoms is not being able to gain weight. If you have hypothyroidism, which would be on the low side, you have a tendency to gain weight easier.

However, please understand that these are not the only symptoms and some can cause some serious problems. If you are concerned, please have your levels, T3, T4, and TSH checked by a doctor.