underactive thyroid
"I didn't even know I had hypothyroid until my Dr ran tests based on family history.  Once it was brought to light, all the symptoms made sense (I had equated them all to weight gain due to change of lifestyle).  I'm still in the process of finding the right dosage, but even on the first legs of that journey I've felt better.   Less lethargic (I'm going to the gym 3 x a week now), less mood swings, a bit better mental clarity, etc.  I haven't noticed nails getting better, but I've always had weak nails.  I have noticed an intense melancholy and inability to focus and stay awake the 1 time I missed a pill.  I'm glad I started this early (29) compared to my poor mom who didn't learn of her deficiency until her late 40's."