Diagnosis:

Doctors diagnose hypothyroidism after completing a history and physical exam of the patient and performing sensitive laboratory tests on the patient's blood. Because symptoms of hypothyroidism can mimic those of many other conditions, blood tests for measuring levels of thyroid stimulating hormone (TSH) and free thyroxine (T4) are the only definitive way to diagnose hypothyroidism. However, the results of these blood tests can be affected by illnesses that are not thyroid related.

Physical Examination

The doctor will check the heart, eyes, hair, skin, and reflexes for signs of hypothyroidism.

Goiter. The presence of a goiter (an enlarged thyroid), especially a rubbery, painless one, may be an indication of Hashimoto's disease. If the thyroid is tender and enlarged but not necessarily symmetrical, the doctor may suspect subacute thyroiditis. A diffusely enlarged gland may occur in hereditary hypothyroidism, in postpartum patients, or from use of iodides or lithium. Goiters may also develop in people with iodide deficiency.

Thyroid Hormone and Antibody Tests

In diagnosing hypothyroidism, blood tests measuring hormone levels are needed to make a correct diagnosis. In some cases, antibody tests are also helpful.

Thyroxine (T4). Hypothyroidism is a condition marked by low thyroxine (T4) hormone levels, and a test can measure levels of this hormone in the blood. However, this test is usually inadequate for the following reasons:

• T4 levels can be normal early in the disease process leading to hypothyroidism. If hypothyroidism is suspected, other tests are needed.
• T4 levels can be low in patients who do not have hypothyroidism. For instance, thyroxine can be extremely variable in very elderly or seriously ill patients and during pregnancy.

Measuring thyroxine is usually performed using a process called a T3 resin uptake to correct for the presence of medications (such as birth control pills, aspirin, and others) that could distort the results. Other tests are needed to confirm a diagnosis of hypothyroidism.

Thyrotropin (Thyroid-Stimulating Hormone or TSH). Measuring TSH is the most sensitive indicator of hypothyroidism. (As with thyroxine levels, however, TSH levels can vary in pregnant women and patients who are ill with other conditions.) In general, results indicate the following:

• TSH levels over 10mU/L. This is a clear indicator of hypothyroidism if T4 levels are low -- and, in most cases, even if they are normal. Patients usually need thyroxine (T4) replacement therapy. They should also be tested for high cholesterol levels and antithyroid antibodies.
• Levels between 4.5 - 10 mU/L. Patients with signs and symptoms of hypothyroidism usually need thyroxine replacement therapy. Patients without symptoms have subclinical hypothyroidism and should be rechecked every 6 - 12 months. Antibody tests may also be performed.
• TSH levels between 0.45 - 4.5 mU/L. These indicate normal thyroid function. (Abnormally low levels suggest hyperthyroidism, which is overactive thyroid.)
• Specific TSH measurement -- even if it is significantly higher than 10 mU/L -- is not associated with the severity of the condition. This can be determined only by measuring thyroxine levels and evaluating the patient's symptoms.

Antithyroid Antibodies. If TSH levels suggest hypothyroidism or subclinical hypothyroidism, the doctor may choose to perform a blood test for specific antithyroid antibodies that act against a factor called thyroperoxidase (TPO). Tests can also check for antibodies to thyroglobulin. Results are particularly helpful in deciding how to treat someone with subclinical hypothyroidism.

About 10% of the American population and 25% of women over 60 years old carry these antibodies, the majority of these women having no thyroid problems. Only about 0.5% have full-blown hypothyroidism, and 10% have subclinical hypothyroidism.

Other Hormone Tests Used for Thyroid Function. Other hormone tests are done if hyperthyroidism is suspected. They include tests for triiodothyronine (T3) and thyroglobulin (also called thyroid binding globulin). Such measurements, however, may also be helpful in detecting sudden temporary increases in thyroid hormone (thyrotoxicosis) that can precede certain forms of autoimmune thyroiditis.

Imaging Tests

Thyroid Scintigraphy. Thyroid scintigraphy, or scan, can be used to determine which areas of the thyroid are producing normal amounts of hormone. The patient drinks a small amount of radioactive iodine or technetium and waits until the substance has passed through the thyroid. Images of a properly functioning thyroid show uniform levels of absorption throughout the gland. Overactive areas show up white, and underactive areas appear dark. Thyroid scans are more likely to be done to evaluate a goiter (swollen thyroid) or thyroid nodules. They can help identify areas of the gland that may have cancer.

Ultrasound. Ultrasound has limited value, but it can visualize the thyroid and specific abnormalities, such as nodules. (It cannot measure the thyroid gland's function, however.)

More Advanced Imaging Tests. If laboratory tests suggest that a pituitary or hypothalamus problem is causing hypothyroidism, the doctor will usually order brain imaging procedures using computed tomography (CT) scans or magnetic resonance imaging (MRI). MRIs may also be used for determining the extent of thyroid cancers and of goiters. MRIs are also being used for investigating hypothyroidism in infants and for determining widespread effects of autoimmune thyroiditis (such as Hashimoto's hypothyroidism).

Needle Aspiration Biopsy

Needle aspiration biopsy is used to obtain thyroid cells for microscopic evaluation. It may be useful to rule out thyroid cancer in patients with thyroid nodules, abnormal findings on a thyroid scan or ultrasound, or those who have a goiter that is large or feels unusual on physical exam. Much like drawing blood, the doctor injects a small needle into the thyroid gland and draws cells from the gland into a syringe. The cells are put onto a slide, stained, and examined under a microscope.

Other Blood Tests

Cholesterol levels need to be checked. Other blood tests may be performed to detect levels of calcitonin, calcium, prolactin, and thyroglobulin and to check for anemia and liver function, all of which may be affected by hypothyroidism.

Screening Recommendations for Hypothyroidism

Screening in Older Adults. Some doctors believe that because thyroid problems are so common in the elderly and thyroid hormone tests are so inexpensive, blood tests for thyroid function should be routine. Undiagnosed hypothyroidism in elderly patients can develop into a serious and even life-threatening situation. Hyperthyroidism also poses many health risks. In fact, during the period around menopause, the symptoms of menopause and hypothyroidism are similar and can easily be confused with each other.

Professional organizations differ widely on screening recommendations. Most do not recommend widespread screening for healthy adults:

• The American College of Physicians recommends that women over 50 years old be screened for thyroid disorders every 5 years. The American Academy of Family Physicians believes that adults do not have to be screened until they are over 60.
• The American Thyroid Association, however, recommends that all adults, both men and women, begin their screening at age 35 and every 5 years thereafter. Experts in this organization argue that such early screening is inexpensive and would prevent progression to hypothyroidism, and therefore possibly heart disease, in people with subclinical hypothyroidism.
• The U.S. Preventive Task Force recommends against routine screening for thyroid disease in adults.

Screening in Pregnant Women. Untreated overt hypothyroidism in a pregnant woman, particularly in the first trimester, may cause premature delivery and birth defects. Birth defects can affect a babyâ ' s intelligence, mental development, and motor skills. Subclinical hypothyroidism also may increase the risk for premature delivery but does not seem to be associated with neurologic or developmental outcomes in children.

Current guidelines recommend targeting screening of women before or during pregnancy based on symptoms or history. Factors that suggest screening is indicated include: History of thyroid disease, goiter, type 1 diabetes or other autoimmune illnesses, history of miscarriages, and history of head and neck radiation or surgery. Women with these factors should have their thyroid checked before pregnancy, or within the first weeks of pregnancy, and should be retested during each trimester.

Screening in Infants. It is very difficult to diagnose hypothyroidism in newborns by symptoms alone. Fortunately, almost all newborns with hypothyroidism are identified shortly after birth through an effective national screening program using a thyroid blood test.

Ruling out Other Disorders

Because the symptoms of hypothyroidism are so similar to common conditions, including aging, diagnosis can be difficult.

Aging-Related Disorders. Some symptoms of hypothyroidism and aging are very similar. Menopausal symptoms often resemble hypothyroidism. Many other problems related to aging -- such as vitamin deficiencies, Parkinson's and Alzheimer's diseases, and arthritis -- also have characteristics that can mimic hypothyroidism.

Depression. A lack of interest in personal relationships, drowsiness, an increase in sleep, slowing of speech, and general apathy are signs of clinical depression as well as hypothyroidism. The two disorders often coexist, particularly in older women, so diagnosing one does not rule out the presence of the other.

Diseases of Muscles and Joints. Joint and muscle aches may be the first symptoms of hypothyroidism. Most likely, however, such pain is not caused by hypothyroidism if other thyroid symptoms remain absent. Numerous conditions can cause muscle and joint pain, and if thyroid levels are normal the doctor should look for other causes.