Hypothyroidism stems from any single cause or a combination of several causes like complications of thyroid gland diseases and autoimmune disorders, exposure to medical treatments, or afflicted brain activities. It commonly arises from a chronic thyroid disease known as Hashiloto's thyroiditis. This thyroid disorder is characterized by enlarged or inflamed thyroid gland, which can ultimately result in severe damage of thyroid cells, if not detected or cured accordingly. The thyroid cells and tissues are damaged because of the antibodies attacking the body's own thyroid tissue, instead of protecting them. Several autoimmune diseases namely, rheumatoid arthritis, diabetes, Vitiligo, Addison's disease, and anemia may also develop into hypothyroidism

Another frequently reported cause of hypothyroidism is exposure to medical treatments like radioactive iodine, lithium, and surgery. An individual who has received some dose of lithium because of a previous or ongoing treatment for a psychiatric disease called manic depressive disorder can likely have an underactive thyroid gland over time. Similarly, a person who has undergone radioactive iodine therapy, which is a kind of treatment for hyperthyroidism and goiter, may also experience manifestations of hypothyroidism. Radioactive iodine can be used to hamper further bulging of the goiter or to prevent secretion of excess hormones by getting rid of these overactive thyroid hormones. When the radioactive iodine therapy causes elimination of more than the needed quantities of cells and tissues, more thyroid tissues are ruined, which lead to hypothyroidism.

Another medical treatment that can possibly cause hypothyroidism is surgery. A person whose thyroid gland has been surgically removed to cure a thyroid gland disease, cancer for instance, can also suffer from hypothyroidism. Since a portion or all of the thyroid gland of that person has already been detached, produced thyroid hormones may be insufficient.

Hypothalamic-pituitary disorders can also instigate hypothyroidism. A person with a perfectly working thyroid gland but defective pituitary gland can suffer from hypothyroidism. When the pituitary gland of a person produces low Thyroid Stimulating Hormone (TSH), which controls the production and release of a thyroid hormone called T4, there will be a deficient activity of the thyroid gland, which in effect, triggers hypothyroidism.

In addition, hypothyroidism stems from childbearing and aging. Recent studies and research have documented that a failing thyroid was found present in individuals above fifty years old, which supports the argument that aging can affect production of thyroid hormones. Pregnant women are also highly susceptible to this thyroid disease because of the hormonal imbalances commonly experienced by expectant mothers. Heredity also accounts for the development of an underactive thyroid gland. This speaks true for individuals suffering from congenital hypothyroidism.