prominent (especially in elderly people), and sometimes symptoms appear to point toward an underactive thyroid even though the thyroid is overactive (see Chapter 19). The most common form of thyroid disease is autoimmune thyroiditis. It probably affects 10 percent of the population of the United States, although only a small fraction of people with this disease actually develop hypothyroidism. Hypothyroidism often begins when a woman is in her 30s. For this reason, and because of the confusion that can exist between the diagnosis and the signs and symptoms a patient experiences, doctors recommend that you start screening for abnormal thyroid function at age 35 and continue at five-year intervals for the rest of your life. This applies to both sexes. Of course, if tests reveal a thyroid condition, your doctor will perform testing much more frequently. Your doctor uses a blood test, the TSH (thyroid-stimulating hormone) test, to screen for abnormal thyroid function. Although the normal range is usually given as 0.5 to 5, the true normal range may be narrower, 0.5 to 2.5 (see Chapter 5). If your doctor tells you that your screening test is normal, but you still have symptoms consistent with hypothyroidism, ask the doctor for the exact number of your TSH. If it’s above 2.5, ask your doctor to consider giving you a trial of treatment with thyroid-hormone replacement.
Checking Thyroid Function As Your Body Changes If you’re taking thyroid hormone treatment, you’re on a fixed dose of medication. However, many physical states, particularly pregnancy (see Chapter 17), create chemical changes in your body that can alter the amount of thyroid hormone that you need to maintain normal function. The same is true as you get older. Chemical changes that cause you to make more thyroid-binding proteins (see Chapter 4) require you to take an increased dose of thyroid medication. Any condition that increases your estrogen is an example, such as pregnancy and taking oral contraceptive pills. As your body makes more thyroid-binding proteins, more of your dose of thyroid is bound to the proteins and less is available to enter your cells. You must increase your dose of thyroid hormone. Blood tests determine when you again have enough. Chemical changes that cause you to make less thyroid-binding proteins require a decreased dose of thyroid hormone. If you take androgens (see Chapter 10) or have a disease that causes your