Why wont my hair extension stay in my hair is it because I have a unattractive thyroid?

Possible. One sign of hypothyroidism is brittle hair. If your hair is breaking wear you apply the extensions this could be a possibility.
Underactive thyroid. An underactive thyroid can make your hair thin, but that should get better with treatment. Best to be propoerly evaluated for your thyroid function and get on treatment if necessary. Otherwise you should ask a hairdresser how to manage your hair extensions.