Hypothyroidism refers to a disorder of the thyroid gland in which insufficient quantities of the thyroid hormones are produced, causing an imbalance in the body's chemical reactions. It is more common in women over the age of 60. 

There are a number of possible causes of hypothyroidism, including thyroid surgery, some autoimmune diseases, pregnancy, an iodine deficiency, congenital disease, or a pituitary disorder. Some medical treatments and medication may also cause hypothyroidism, such as radiation therapy and treatment for hyperthyroidism (an excess of thyroid hormones).

Symptoms of hypothyroidism include gaining weight; fatigue; constipation; depression; swelling of the hands, feet or face; thinning hair; dry skin; muscle weakness; stiffness or pain in the joints; impaired memory; and increased cholesterol levels in the blood. Hypothyroidism may lead to a variety of other health problems, including heart problems, mental health issues, peripheral neuropathy, infertility, and goiters. 

Treatment of hypothyroidism usually involves the daily use of synthetic thyroid preparations to restore normal hormone levels.