The symptoms and effects of hypothyroidism can vary greatly depending on the age and sex of the affected individual. The lack of thyroid hormone in an infant with untreated congenital hypothyroidism may cause cretinism, a condition resulting in severe growth delay and mental retardation. Routine screening for hypothyroidism in all newborns by a heel prick blood test has decreased the incidence of cretinism.
In older children, signs of hypothyroidism can include unexplained daytime fatigue, changes in school grades, difficulty concentrating, and inattentiveness in school. Sometimes, however, an unexplained change in growth rate is the only evidence of thyroid failure in children.
In women of reproductive age, hypothyroidism can impair fertility and increase the risk of miscarriage during the second trimester. Other symptoms may include a goiter (an enlarged thyroid gland), fatigue, weight gain, mood swings, depression, dry skin, brittle hair, fluid retention, muscle weakness, and constipation.
In the elderly, the symptoms can be very vague and are often mistaken for depression or dementia, since thyroid failure can cause sleep disturbance and poor memory.
Worldwide, the most common cause of hypothyroidism is dietary iodine deficiency. However, since iodine has been added to salt and other foods in the United States, hypothyrodism caused by iodine deficiency is very rare among Americans. The most common cause of hypothyroidism in the United States is an inherited autoimmune condition called Hashimoto thyroiditis, which affects up to 14 million people.
Autoimmune disorders are caused when the body’s natural defenses against “foreign” or invading organisms begin to attack healthy tissue for unknown reasons. These natural defenses use proteins called antibodies. Hashimoto thyroiditis produces antibodies that damage the thyroid tissue, resulting in thyroid failure.
Other common causes of hypothyroidism include the surgical removal of the thyroid, exposure to radiation such as radioactive iodine therapy, or external radiation used to treat some forms of cancer. Certain drugs such as lithium carbonate (for treatment of bipolar disorder) and amiodarone (brand name Cordarone, a drug prescribed for certain heart conditions) can cause hypothyroidism. Rarely, abnormalities of the hypothalamus or pituitary gland may result in hypothyroidism.
Hyperthyroidism. Overactive thyroid, or hyperthyroidism, is less common than hypothyroidism. It tends to affect all age groups equally, but it is nine times more common in women. Like hypothyroidism, the symptoms of hyperthyroidism are varied and can be vague, particularly if the disease progresses slowly over a prolonged period. In children, it can result in accelerated growth, hyperactivity, loss of concentration, poor handwriting, and short-term memory loss. In young women, it may result in absent or irregular periods, recurrent miscarriages, and infertility.
In older women, hyperthyroidism can cause hot flashes, mood swings, sweating, and weight changes, symptoms that may be confused with signs of menopause. Other typical symptoms of hyperthyroidism include irritability, fast heart rate, weight loss despite increased appetite, frequent bowel movements, insomnia, decreased energy level, and shaky hands. In addition, a goiter may be present. (To learn more about symptoms of thyroid disorders, see “Symptoms and Signs.”)
The most common cause of hyperthyroidism in people under age 40 is Graves disease, a type of autoimmune thyroid disease. In this case, the antibodies stimulate the thyroid to enlarge and overproduce thyroid hormone. Some people with Graves disease develop eye problems that can cause the eyes to protrude and that may threaten vision. Other causes of hyperthyroidism include thyroid nodules, which are overactive (usually noncancerous) growths in an otherwise normal thyroid gland, and thyroiditis, or inflammation of the thyroid gland. (See “Suggested Reading” for more information about thyroid disorders.)