I thought i had an under active thyroid but blood tests clear

over the last 6 months i have been getting increasingly more exhausted. By 2 in the afternoon i am worn out and could sleep until the next day and still be tired. I have also steadily put on weight although i am healthier than i have every been and go to the gym regularly. In 6 months I have gone from 9 stone to 12 stone and am very worried. I was sure i had an under active thyroid as my nan and aunty both do, but i have had blood tests and it is showing clear. I am getting more and more depressed as this tiredness and putting on weight is affecting my job and general happiness. what else could this be?