The most common cause of low thyroid levels is a condition known as Hashimoto's thyroiditis, which leads to the inflammation of the thyroid gland, according to WebMD. This is an autoimmune disorder in which the body produces antibodies that attack the thyroid. Viral infections can also cause low thyroid levels.
Other causes of low thyroid levels include radiation therapy, radioactive iodine treatment, certain medications and thyroid surgery, explains WebMD. An iodine deficiency, pregnancy and damage to the pituitary gland may also cause low thyroid levels. Women are more likely to have low thyroid levels than men, and individuals are more likely to develop low thyroid levels if they have a family history of autoimmune conditions.
There are a number of symptoms associated with low thyroid levels, such as changes in the menstrual cycle, depression and constipation, according to WebMD. Dry hair, dry skin, fatigue, sensitivity to cold and a slow heart rate are also associated with low thyroid levels. Swelling of the thyroid gland, unexplained weight gain and carpel tunnel syndrome may also indicate that a person has low thyroid levels. The main treatment for this condition is a synthetic thyroid hormone called T4, which thyroid patients take every day. Regular blood tests are also performed to test the thyroid levels in the body.