Hello,
This is a common problem with many individuals so you need to worry much regarding this.
See, to gain weight is an individual phenomenon. Meaning, it depends upon the physiology of the individual and it is different in everyone.
There is some ideal weight according to body height but that is not the absolute thing.
The prime aim of eating is to provide body with enough nutritious material so as to provide body with the raw material to maintain integrity and immunity.
You eat much enough to suffice the need of the body.
If you are disease free and sound in health then it doesn't matter of you don't gain weight.
Yes life style of course counts in these things.
Sound sleep, proper timely nutritious protein diet and exercise would definitely help.
Exercise make you body that you need to increase muscle mass and at the same time if you are providing enough nutrition, it will take up those.
Hyper thyroidism has got many other symptoms than this.
You definitely can go with
yoga too, if order to re-establish the normal functioning of your body if at all it is disturb.
Hope it is helpful.
Thanks.