Hypothyroidism Diagnosis

Tests that Detect Thyroid Dysfunction

Hypothyroidism can be an elusive disease. Its presence can be so slight that you may not even recognize that anything is wrong. On the other hand, the symptoms may be undeniable—dry skin, intolerance to cold, an overwhelming feeling of fatigue. Whatever the case, if you have any suspicion that you may be hypothyroid, it's important to talk to your doctor about getting tested. Diagnosing hypothyroidism early on will provide you with earlier access to treatment, which may have a significant and positive impact on your quality of life.

Hypothyroidism is common in women, particularly those over age 50, and the chances of a woman developing hypothyroidism are increased during pregnancy, after she gives birth, and around the time of menopause.

TSH Testing
In diagnosing hypothyroidism, your doctor will take into account both your symptoms and the results of a thyroid stimulating hormone (TSH) test. In essence, hypothyroidism is the result of diminished levels of thyroid hormones—known as T3 and T4. Your pituitary gland stimulates the release of these hormones into the bloodstream through TSH, so it makes sense that the first line of testing for hypothyroidism is based on your levels of TSH. Your doctor will measure your TSH levels with a blood test.

TSH levels are determined by ranges (all figures in mU/L—milliunits per liter). Below are the ranges, according to the American Thyroid Association. However, these numbers are not set in stone. Normal ranges can vary by individual, and they can even change over the course of a day. Also, ranges can vary slightly from lab to lab:

  • 0.4: normal
  • 2.5: at risk
  • 4.0: mild hypothyroidism
  • 10.0: hypothyroidism.

Today's TSH tests are very accurate and sensitive; they can help diagnose even the mildest cases of hypothyroidism.

T4 Testing
It's important to understand that just because your TSH test comes back normal, it does not rule out the possibility of you being hypothyroid. If your symptoms still point to a hypothyroid diagnosis, your doctor may measure the level of free T4 (the portion of total T4 thyroid hormone that is available to your tissues) in your bloodstream.

Many people who are hypothyroid actually have high levels of TSH and low levels of T4—your doctor may refer to this as "primary hypothyroidism." This occurs because the pituitary gland has recognized that the thyroid isn't producing enough hormones. As a result, the pituitary releases more TSH in an effort to stimulate the thyroid into producing hormone. But if the thyroid isn't working properly, it won't react to the signals from the pituitary. This leads to high levels of TSH and low levels of free T4. If your free T4 levels are too low—the normal range for T4 is 5 to 13.5 micrograms per deciliter—it's a possible indicator that you have hypothyroidism, even if your previous TSH tests came back normal. Again, the normal ranges vary from lab to lab.

Anti-thyroid Microsomal Antibodies Testing
A third hypothyroid test is for anti-thyroid microsomal antibodies—anti-thyroid peroxidase (anti-TPO). These antibodies, which are produced by the immune system, may attack thyroid cells. If a blood test determines their presence, it shows that there has been thyroid damage which could potentially lead to hypothyroidism. This autoimmune damage characterizes Hashimoto's thyroiditis. Most primary care physicians do not test for anti-thyroid microsomal antibodies, so your doctor may refer you to a specialist for this particular test. These patients could also have an initial hyperthyroidism type of picture at the destruction phase of thyroiditis when the thyroid hormones stored in the cells are secreted out of the cell.

If you believe you are at risk for hypothyroidism, don't hesitate in discussing various testing options with your doctor. Diagnosing hypothyroidism early on is an essential part of putting you on a successful treatment path.

View Sources
  • ATA Hypothyroidism Web Booklet. American Thyroid Association Web site. Available at: http://thyroid.org/patients/brochures/Hypothyroidism%20_web_booklet.pdf. Accessed June 10, 2009.
  • Mayo Clinic Hypothyroidism Tests and diagnosis page. Mayo Clinic Health Information Web site. Available at: http://www.mayoclinic.com/health/hypothyroidism/DS00353/DSECTION=tests-and-diagnosis. Accessed June 10, 2009.
  • Skugor, M. The Underactive Thyroid: Hypothyroidism. In: The Cleveland Clinic Gide to Thyroid Disorders. New York: Kaplan Publishing; 2009:17-21.