What Tests Will Confirm the Diagnosis of Hypothyroidism?

September 1998
A look at thyroid function tests and other measures used to diagnose hypothyroidism.

Advances in diagnostic methods now make it possible to detect hypothyroidism in almost all cases before severe symptoms develop. Physicians can make the diagnosis of hypothyroidism after completing a history and physical exam of the patient and performing sensitive laboratory tests on the patient's blood. Some physicians 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. One study reported that in elderly patients who require emergency surgery and have undiagnosed hypothyroidism, the consequences can be very serious, even fatal. The American College of Physicians now recommends that women over 50 years old should be screened every five years. Researchers in one study, in fact, reported that screening men and women at age 35 and every five years afterward would be cost-effective and would prevent progression to hypothyroidism in people with mild thyroid failure but no symptoms (subclinical hypothyroidism). It might also help prevent unhealthy cholesterol levels in such people. Other experts feel, however, that cholesterol levels can be lowered through other means and that there is no evidence that treating people who have only mildly abnormal thyroid levels and no symptoms would improve their lives.

Physical Examination

A goiter (an enlarged thyroid) may be evident on examination. A rubbery, painless goiter may be an indication of Hashimoto's disease. If the thyroid is tender and enlarged but not necessarily symmetrical, the physician may suspect subacute thyroiditis. A diffusely enlarged gland may occur in hereditary hypothyroidism, in postpartum patients, or from use of iodides or lithium. The physician will check the heart, eyes, hair, skin, and reflexes.

Measuring Thyroid Hormone Levels

Thyroid-stimulating hormone (TSH) and thyroxine (T4) levels are usually both measured using blood samples, although TSH is the more sensitive indicator of hypothyroidism. Its function is to stimulate thyroxine production when levels drop, and so the pituitary gland secretes more TSH as soon as it senses even slight reductions in thyroxine levels. In fact, thyroxine may still be within normal range when the pituitary begins to increase the supply of TSH. If TSH levels are elevated above 6 mU/L regardless of thyroxine levels -- the physician can still make a diagnosis of hypothyroidism, although the condition is considered to be subclinical if thyroxine levels are normal and the patient has no symptoms. In the very elderly or seriously ill patients and during pregnancy both thyroxine and TSH levels may be extremely variable; in such patients measurements of the hormones should be repeated before starting thyroid-hormone therapy. Tests called sensitive thyroid-stimulating hormone (sTSH) assays and so-called free thyroxine (fT4) assays have been developed and are believed to be very accurate; one study reported that sTSH was so accurate that the fT4 test was not needed.

Childhood Screening

Almost all newborns with hypothyroidism are identified shortly after birth through an effective national screening program using a thyroid blood test. Each year over 1,500 children are now saved from subnormal intelligence.

Testing during Pregnancy

Because untreated hypothyroidism is a serious problem for the unborn child, all pregnant women should be tested for thyroid function. Elevated levels of estrogen during pregnancy cause thyroid hormone levels to rise. Therefore, a pregnant women with an underactive thyroid may have what appears to be normal levels of thyroid hormones, but she may actually be hypothyroid. A blood test showing elevated TSH levels, however, is a reliable indicator of an underactive thyroid, even in pregnancy.

Antithyroid Antibodies

A blood test for antithyroid antibodies is sometimes used to detect Hashimoto's thyroiditis, particularly in patients who have knobby goiters. If high levels of antibodies are present Hashimoto's thyroiditis is a certain diagnosis. Even if patients have no symptoms at the time of the test, a positive result usually means that a patient has a 4% to 8% chance of developing symptoms each year.

Imaging Tests

Imaging procedures including x-rays and ultrasound may be used to visualize the thyroid, but they do not measure the thyroid gland's function. Thyroid scans are then used to determine whether the thyroid is producing normal amounts of hormone. The patient drinks a small amount of radioactive iodine or technetium and waits until it has been through the thyroid. Images of a properly functioning thyroid would show uniform levels of absorption throughout the gland. Overactive areas would show up white and underactive areas would appear dark. Thyroid scans are usually unnecessary unless the physician needs to rule out suspected cancer.

If laboratory tests suggest that a pituitary or hypothalamus problem is causing hypothyroidism, the physician will usually order brain imaging procedures using computed tomography (CT) scans or magnetic resonance imaging (MRI).

Needle Aspiration Biopsy

Needle aspiration biopsy is a common procedure performed in a doctor's office and used to obtain thyroid cells for microscopic evaluation. Much like drawing blood, the physician 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

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

Well-Connected Board of Editors

Harvey Simon, M.D., Editor-in-Chief
Massachusetts Institute of Technology; Physician, Massachusetts General Hospital

Masha J. Etkin, M.D., Gynecology
Harvard Medical School; Physician, Massachusetts General Hospital

John E. Godine, M.D., Ph.D., Metabolism
Harvard Medical School; Associate Physician, Massachusetts General Hospital

Daniel Heller, M.D., Pediatrics
Harvard Medical School; Associate Pediatrician, Massachusetts General Hospital; Active Staff, Children's Hospital

Irene Kuter, M.D., D. Phil., Oncology
Harvard Medical School; Assistant Physician, Massachusetts General Hospital

Paul C. Shellito, M.D., Surgery
Harvard Medical School; Associate Visiting Surgeon, Massachusetts General Hospital

Theodore A. Stern, M.D., Psychiatry
Harvard Medical School; Psychiatrist and Chief, Psychiatric Consultation Service, Massachusetts General Hospital

Carol Peckham, Editorial Director

Cynthia Chevins, Publisher

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