Some people do lose weight or can't gain weight hypo or hyper...just lucky in the gene department, I guess!
So, with the range you provided, you are a little higher in the range than I'd tought at first...quite a bit over midrange. With an FT4 that high, I'd expect your TSH to be lower than it is. Your TSH is in hypo territory, but your FT4 by no means is. However, TSH is volatile and swings a lot even intraday.
It might be worthwhile to request another set of labs. I'd request a repeat of FT4 and TSH and ask the doctor to add FT3 this time. Since your sister has Hashi's, it might also be a good idea to test thyroid antibodies...TPOab and TGab. If either of those is elevated, autoimmune thyroid disease (Hashi's or Graves') is indicated. Since autoimmune disease tends to run in families (but often not the SAME autoimmune disease), you might also ask for TSI, which is the definitive test for Graves'. Those would give you a much better idea of what's going on with your thyroid.
Does your PCP have a theory as to what's causing your symptoms?
I was tested for diabetes. The results came back normal.
My doctor (general practitioner) has said that everything looks normal. The nearest endocrinologist is quite a ways away from where I'm currently living. So that's why I'm trying to get this worked out through my GP.
I'm not sure the cause of my grandmother's thyroid problems.
My aunt's hypothyroidism is caused by a growth on her thyroid.
My sister's hypothyroidism is caused by Hashimoto's (says her endocrinologist). She also has trouble putting on weight and is incredibly skinny. So the weight-factor may just be genetic. All of her other symptoms, however, fit those of hypothyroidism.
Have you been checked for diabetes. Inability to gain weight, excessive thirst, mood swings, along with irrational behavior are major symptoms.
What has your doctor said so far as an explanation of this?