Hypothyroidism is caused from decreased production of thyroid hormone, which results in decreased metabolism. Hypothyroidism occurs more commonly in women 1.2-2% as compared to men 0.2 %. Hypothyroidism is also more common in menopausal women. People at high risk for thyroid dysfunction include post-partum women, people with high levels of radiation exposure (< 20mGy), elderly, and people with Down Syndrome. The elderly are often undiagnosed because symptoms mimic aging. Also as a person ages, the incidence increases. The primary cause of hypothyroidism is the failure of the thyroid gland termed: Primary Hypothyroidism.

There are several causes of primary hypothyroidism such as Hashimoto's disease (inflammation of the thyroid by an autoimmune mechanism), iatrogenic hypothyroidism such as after radioactive iodine therapy, iodine deficiency, enzyme defects, underdevelopment of the thyroid gland, and substances that cause goiters. Another cause of thyroid disorder is Wilson's Syndrome. A less common cause of hypothyroidism is from pituitary or hypothalamic disease called Secondary Hypothyroidism.

In hypothyroidism, patients can have a wide variety of symptoms but generally there is slowing of metabolic processes. There is a range of hypothyroidism that consists of subclinical to overt hypothyroidism to myxedema. Patient's who are older have fewer signs and symptoms and in both young and old patients there is little correlation between clinical and biochemical manifestations.