According to Wikipedia, in areas of the world with sufficient dietary iodine, hypothyroidism is most commonly caused by the autoimmune disease Hashimoto's thyroiditis (chronic autoimmune thyroiditis), a disease in which the thyroid gland does not produce enough thyroid hormone. It can cause a number of symptoms, such as poor ability to tolerate cold, a feeling of tiredness, constipation, depression, and weight gain. (Wikipedia page edited July 22, 2017)
[PMID 22654557] A 2011 article called "Hashimoto's Thyroiditis: From Genes to the Disease" explained that "Hashimoto’s thyroiditis (HT) is the most prevalent autoimmune thyroid disorder", and is "recorded in 4% to 9.5% of the adult population." In the progress of this disease, "Intrathyroidal lymphocytic infiltration is followed by a gradual destruction of the thyroid gland which may lead to subclinical or overt hypothyroidism." ... Genes for human leukocyte antigen (HLA), cytotoxic T lymphocyte antigen-4 (CTLA4), protein tyrosine phosphatase nonreceptor-type 22 (PTPN22), thyroglobulin, vitamin D receptor, and cytokines are considered to be of utmost importance."