Right now, a battle is waging in the endocrinology community regarding the so-called "reference range" for the Thyroid Stimulating Hormone (TSH) test. The importance of this controversy cannot be underestimated. The majority of practitioners -- including endocrinologists, the physicians who specialize in thyroid disease -- rely solely on the TSH test as the primary test, the supposed "gold standard" in fact, for diagnosing and managing most thyroid conditions.
There is ongoing controversy about whether reliance on the TSH test -- to the exclusion of clinical symptoms and other tests such as Free T4, Free T3, and antibodies tests -- is medically sound. That is a controversy that is unlikely to be decided for years. The situation today, however, is that the majority of physicians do rely almost exclusively on the TSH test to detect thyroid disease, and monitor the effectiveness of treatment.
Surprisingly, however, while the medical community does rely on the TSH test, there is complete disagreement within the community as to what constitutes the "normal range."
What is a Reference Range?
Reference range is a critical component, and the validity of the entire TSH test as diagnostic tool depends on it. A TSH reference range is obtained by taking a large group of people in the population, measuring their TSH levels, and calculating a mean value. Supposedly, these people should be free of thyroid disease, so that the level represents the mean TSH of a typical thyroid disease-free person in the population. The reference range is what determines whether or not thyroid disease is even diagnosed at all, much less treated, and when it is diagnosed, how it is treated.
Currently, at most laboratories in the U.S., the reference range for TSH tests is approximately 0.5 to 5.0. Depending on the lab, you may seem some variations, i.e., 0.4 to 5.5, or 0.6 to 5.7, etc., but generally, 0.5 to 5.0 is considered typical of many labs.
Typically, doctors interpret levels below 0.5 as indicative of hyperthyroidism (an overactive thyroid), and levels above 5.0 as indicative of hypothyroidism (an underactive thyroid.)
Changing the Reference Range
After noticing that patients who had TSH levels in the higher end of the normal range tended to go on to develop hypothyroidism more often than those in the lower end of the spectrum, researchers delved more fully into understanding the validity of the reference ranges in use. They found that the upper TSH normal range has traditionally included people who have mild thyroid disease, and their higher TSH levels skewed the standard curve, potentially making the reference range wider than it should be, and excluding some people who legitimately had a thyroid condition.
These findings led to the recommendation in January 2003 by the American Association of Clinical Endocrinologists (AACE) that doctors "consider treatment for patients who test outside the boundaries of a narrower margin based on a target TSH level of 0.3 to 3.0. AACE believes the new range will result in proper diagnosis for millions of Americans who suffer from a mild thyroid disorder, but have gone untreated until now."
In a statement from the AACE, Hossein Gharib, MD, FACE, and president of AACE at the time, said, "The prevalence of undiagnosed thyroid disease in the United States is shockingly high...The new TSH range from the AACE guidelines gives physicians the information they need to diagnose mild thyroid disease before it can lead to more serious effects on a patient's health - such as elevated cholesterol, heart disease, osteoporosis, infertility, and depression."
AACE cited as evidence the guidelines issued by the National Academy of Clinical Biochemistry, part of the Academy of the American Association for Clinical Chemistry (AACC), and presented in their Laboratory Medicine Practice Guidelines for the Diagnosis and Monitoring of Thyroid Disease. Late in 2002, the group concluded that "it is likely that the current upper limit of the population reference range is skewed by the inclusion of persons with occult thyroid dysfunction." In their guidelines, the National Academy of Clinical Biochemistry reported that: "In the future, it is likely that the upper limit of the serum TSH euthyroid reference range will be reduced to 2.5 mIU/L because 95% of rigorously screened normal euthyroid volunteers have serum TSH values between 0.4 and 2.5 mIU/L." They also stated that "a serum TSH result between 0.5 and 2.0 mIU/L is generally considered the therapeutic target for a standard L-T4 replacement dose for primary hypothyroidism."