As of February 2010, at most laboratories in the U.S., the official "normal" reference range for the Thyroid Stimulating Hormone (TSH) blood test runs from approximately .5 to 4.5/5.0.
Reference range is a critical component, and the validity of the entire TSH test as diagnostic tool depends on it. TSH reference range is what determines -- for the vast majority of physicians, who rely on blood tests almost exclusively -- whether or not thyroid disease is even diagnosed at all, much less treated, and when diagnosed, how it is treated.
A 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. What experts are now coming to understand, however, is that the upper range in the TSH normal reference range has included people who actually have mild or developing thyroid disease, and their higher TSH levels skewed the standard curve.
This understanding led to the recommendation in January 2003, by the American Association of Clinical Endocrinologists, 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."
This was backed up by research done 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, this group 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 more than 95% of rigorously screened normal euthyroid volunteers have serum TSH values between 0.4 and 2.5 mIU/L."
Researchers also looked at an important question: If the normal TSH range were narrowed, as has been recommended by AACE and the National Academy of Clinical Biochemistry, what are the implications?
One study found that using a TSH upper normal range of 5.0, approximately 5% of the population is hypothyroid.
However, if the upper portion of the normal range was lowered to 3.0, approximately 20% of the population would be hypothyroid.
Implications for Patients
It's now nearly a decade since the experts recommended this new normal range, doctors have changed their positions yet again, and the the guidelines are still in flux. Some doctors use the new range for diagnosis and therapeutic management -- others refuse to consider anything unless it's marked "High" or "Low" on laboratory reports.
In the meantime, patients can patronize those doctors who stay up on the research, and leave behind those doctors who stick their heads in the sand and refuse to recognize millions of undiagnosed, undertreated people with hypothyroidism.
More Information on the Controversy
- Endocrinologists Say TSH Normal Range is Now 0.3 to 3 / New Guidelines Say Millions More Are at Thyroid Risk
- Does Your Doctor Know About the New TSH Lab Standards?
- TSH Reference Range Wars: What's "Normal?", Who is Wrong, Right?
- The TSH Normal Range: Why is There Still Controversy? Interview With Leading Endocrinologist, Dr. Jeffrey Garber
Source: Fatourechi V, Klee GG, Grebe SK, et al. Effects of reducing the upper limit of normal TSH values. JAMA. 2003;290:3195-3196.