Why does your hair fall out with low thyroid?

Hair grows slower.: That's a tough question. The answer is that your hair is just growing slower. Hair has 3 phases of growth. It grows, then it stops growing and then a new hair grows in and the old hair falls out. That is why you always have some short hairs. With low thyroid, the hair will fall out but the new hair takes longer to grow in or doesn't grow. So the overall effect is thinning hair.