Levothyroxin brands and how each differs

[HealthySenior]

I am basically a healthy senior. I do take levothyroxin .75 mcg. However, I am really confused after reading several blogs about two different brands of levothyroxin. My doctor has never mentioned two different brands. I also read that one person had allergies and took one brand and then another to help them. Before taking levothyroxin for hypothyroidism, I was tired and had gained weight. Also my hair was thinning. As soon as I started taking it my hair decreased and my weight went way down to 100 lbs. I fell backwards about a year or so ago and really cracked the back of my head hard, now the hair is simply not growing there. I am wondering if it ever will grow again? Has anyone ever experienced something like this.