I’ve always been the “normal” one in the family, but the last 3 years I feel like I’ve started falling apart, and I’m only 23!

I have signs of Hypo-thyroidisim, but have been tested 3 times and they say I’m in “normal” range. My mom was diagnose when she was 22 once she began gaining major weight after the birth of my sister in 1983. She's the one who encouraged me to be tested.
I am currently taking~
100 mg Zoloft per day for depression/social anxiety
100 mg Nitrofurantoin MONO/MAC (generic for
Macrobid) for Trigonitis
Here are my Symptoms (I understand not all of these are hypo-thyroid symptoms)
· Low Basal Body Temperature (I’ve been charting, we're useing the FAM)
· Losing my hair
· Dry Skin (face dry too, but semi-oily T-Zone)
· Stiffness and minor pain after long periods of sitting/laying. I saw my doctor and after a few x-rays, I was diagnosed with minor arthritis and scoliosis in my lower back.
· FATIGUE
· Depression
· Low immune system
· Impatient/irritable
· Weight Gain/Over Weight
· Light sensitivity
· Poor mental function
I don't know what to do anymore, and am at the end of my rope. Any help would be greatly appreciated! I saw someone say that we should have our B12 checked, so I've got that on my list.