I don't think the loss of your brows is much to do with a thyroid problem - apparently thinning brows and losing the tail is part of the aging process! As is thinning hair.
I have one eyebrow a lot thinner than the other after I had a fall and split my forehead open from my brow to hairline over my left eye.
I have invested in a decent brow powder to balance it out and use a caffeine based shampoo that seems to be working.
I have heard that smoothing on a small amount of castor oil along your brows at night is supposed to thicken your brows, but haven't tried that yet!