In various thyroid conditions, a side effect can be hair loss. In Hyperthyroidism there is an excess of thyroid hormone in the blood stream that can cause an overactive thyroid condition. When there is a deficiency of the thyroid hormone in the blood stream it is known as hypothyroidism. There are also other autoimmune disorders that make antibodies that will attack the thyroid gland. These can also cause hypothyroidism.
All of these conditions may contribute to thinning of the hair and may cause it to be brittle and break off.