Lost all my hair on three separate occasions. Doctor diagnosed B-12 deficiency and anemia. Started B-12 sublingual - Methylcobalamin (the kind of B-12 which doesn't need to be digested through your stomach - absorbed under your tongue) with B-6, folic acid and Biotin. My ferritin (stored iron) levels were plenty high enough to sustain hair growth.
Amazing how that little vitamin B-12 changed 8 separate blood factors that were bad on my CBC blood panel and my hair has grown back just as thick. It did take about 6 months to stop feeling so tried. Without the B-12, you won't have the iron in your blood cells that you need for everything else.
Now, I also found out I have disseminated dwarf tapeworms. The disseminated dwarf tapes sucker onto the ends of my eyelashes and loves my eyebrows. I keep losing areas of my eyebrows. Bought a microscope and couldn't believe my eyes - dwarf tapeworms. I'm now on MattK3's protocol and am much better, but still in the middle of it so, time will tell. I have found OccuSoft Plus lid wipes are so good. Use them in the morning and evening - I cut one pad into 4 pieces so they last longer. If you have a jeweler hoop and look at the pads after you use them on your eye lid and eyebrow, you can see the parasites. They get caught on the pad.
It could be thyroid, but because of hair loss, I would also ask the doctor for a CBC, to check ferritin, TIBC, and Iron levels.