Hypothyroidism

Hypothyroidism, or an underactive thyroid gland, is a decrease in the production of thyroid hormones, which slows vital body functions.

More than 15% of older people have some degree of hypothyroidism. Women are affected about twice as often as men.

Causes

Hypothyroidism can have several causes, although in many people a specific cause cannot be found. In Hashimoto's thyroiditis, the most common specific cause of hypothyroidism, something triggers the body's immune system to produce cells called lymphocytes that attack the thyroid gland. At first, the gland may enlarge. Eventually, Hashimoto's thyroiditis damages the thyroid gland and can leave it unable to make enough hormones, resulting in hypothyroidism.

Other causes of hypothyroidism include previous radiation to and surgical removal of the thyroid gland. Much less commonly, hypothyroidism results from disorders of the hypothalamus or pituitary gland. Certain drugs, such as lithium (which is used to treat bipolar disease), drugs used to treat overactivity of the thyroid gland, and drugs that contain iodine (such as amiodarone, which is used to treat heart disease), can cause hypothyroidism as well.

Symptoms

In older people, hypothyroidism can cause confusion, decreased appetite, weight loss, sensitivity to cold, constipation, joint stiffness, dizziness, and a tendency to fall. Some people feel tired, weak, or depressed. The skin may become dry and coarse, and the face may become puffy and swollen, especially around the eyes. Muscles and joints may ache. Weakness may progress, interfering with the ability to walk. In its most severe form, hypothyroidism can cause a person to slip into a coma (myxedema coma) and may even be fatal.

Doctors may not recognize these symptoms as being caused by hypothyroidism: the symptoms may be subtle and vague and are common among older people who do not have hypothyroidism. Symptoms of hypothyroidism that are common among younger and middle-aged people, such as weight gain, muscle cramps, tingling, and the inability to tolerate cold, are less common among older people, and when they do occur among older people, they are less obvious.

Diagnosis and Screening

Doctors feel (palpate) the thyroid gland, which may be enlarged. During a physical examination, doctors may find some evidence of hypothyroidism. The area around the eyes is checked for puffiness, and the skin is checked for dryness. Doctors tap the knees, ankles, and elbows to see if reflexes are slow. Body temperature may be slightly decreased. Often, however, the examination findings are normal.

If hypothyroidism is considered a possibility, blood tests are usually done. The level of thyroid-stimulating hormone is measured first. If hypothyroidism is present, the level of thyroid-stimulating hormone is almost always high. In rare instances in which hypothyroidism is due to a disorder that affects the pituitary gland rather than the thyroid gland, the level of thyroid-stimulating hormone is normal or low. If the level of thyroid-stimulating hormone is normal and doctors still suspect hypothyroidism, they measure the level of T₄ hormone. A low level confirms the diagnosis of hypothyroidism. Other tests may be needed to determine the cause.

Screening older people for hypothyroidism is helpful. Many experts recommend measuring the level of thyroid-stimulating hormone in the blood every 5 years. The level should be measured more often in people taking certain drugs (such as lithium or amiodarone), in those who have had a thyroid problem in the past, in those who have a family history of thyroid disorders, and in those who have conditions involving the immune system. Thyroid testing should also be done in people who have high cholesterol levels, which can be caused by an underactive thyroid gland.

Treatment and Outlook

People who have hypothyroidism need to take thyroid hormone to replace the hormones that the thyroid gland is no longer making. Treatment usually begins with a small dose of thyroid hormone. The dose is then slowly increased about every 4 weeks. The body may take that long to fully adapt to each change in dose. To reduce the risk of side effects from treatment, people who have heart disease or other serious disorders begin treatment with yet a smaller dose, which is increased even more slowly.

After about 3 to 4 months, the appropriate dose of thyroid hormone is usually reached. After this point, the dose occasionally needs to be adjusted, depending on weight loss or gain and the use of certain drugs. Sometimes certain drugs (such as iron supplements, calcium carbonate, or aluminum-containing antacids) can interfere with the absorption of thyroid hormone. Certain other drugs (especially drugs used to prevent seizures, such as carbamazepine) increase the body's metabolism of thyroid hormone. When a drug interferes with absorption or speeds up metabolism of thyroid hormone, an alternative drug may be taken or a higher dose of thyroid hormone may be needed. To help determine if the person is responding well to treatment, doctors periodically measure the level of thyroid-stimulating hormone.

Treatment is very successful in eliminating or greatly reducing the symptoms of hypothyroidism. However, treatment with thyroid hormone does not cure the problem that caused the hypothyroidism. For this reason, treatment with thyroid hormone almost always has to be continued for life.