Hypothyroidism can be difficult to diagnose because its symptoms vary considerably and may be found in other diseases. This illness is most common in women over the age of 50. As a result, physicians often screen for this disease during routine examinations of women in this age group. Screenings also may be performed on women who are pregnant or who are thinking of becoming pregnant.

Screenings also may be performed on women who are pregnant or who are thinking of becoming pregnant. The American Academy of Clinical Endocrinologists recommends thyroid blood testing before pregnancy or during the first trimester, although the American College of Obstetricians and Gynecologists now advises against this routine screening.

Infants are routinely checked with a blood test shortly after birth for congenital hypothyroidism. People with a family history of hypothyroidism also may be good candidates for regular screenings.

In other cases, a physician may check for hypothyroidism in patients who display symptoms of this illness. Diagnosis typically begins with the physician reviewing the patient's medical history and performing a physical examination. The physician will look for signs such as changes in the thyroid gland, dry skin, swelling, slowed reflexes and slowed heart rate.

With hypothyroidism, thyroid blood tests usually show high levels of thyroid-stimulating hormone (TSH) and low measures of thyroxine (T4). The physician may also check the blood for the antibodies that are associated with Hashimoto’s thyroiditis. Patients with a condition called subclinical hypothyroidism, which does not cause prominent symptoms but can still cause health problems, typically have high levels of TSH and normal levels of thyroxine.

When blood testing reveals hypothyroidism, the physician will probably suggest a follow-up test that measures the level of a thyroid hormone in a patient’s system. Tests also may be performed to look for complications associated with hypothyroidism. For example, a chest x-ray may show an enlarged heart, and a cholesterol blood test may show unhealthy levels of cholesterol.