Hypothyroidism (under active thyroid gland) is a very common condition and in the US, is most often caused by a group of disorders that results in antibodies being produced in the body that attack the thyroid gland. Autoimmune Thyroid Disease (AITD) is much more common in women and Grave's Disease (causing overactive thyroid gland) and Hashimoto's Thyroiditis (resulting in under-activity) are common examples. Antibodies produced by the immune system are supposed to only target 'foreign' proteins or antigens, but when the antibodies start confusing the body's own tissues (like thyroid or skin or joints) for something foreign then this will lead to inflammation of the the thyroid (thyroiditis) or skin (vitiligo) or joints (rheumatoid arthritis). It is also true that people who have one type of 'auto' antibody, there is a higher chance they will develop other antibodies directed against different tissues. When antibodies are directed against the parietal cells of the the stomach, they can cause a condition called Atrophic Gastritis which will diminish or eliminate production of acid and Intrinsic Factor leading to vitamin B12 deficiency. Atrophic gastritis has been found to be as common as 35% - 40% of people with Autoimmune Thyroid Disease (AITD) and many experts recommend that thyroid patients be checked for vitamin B12 deficiency.