Thyroid Testing: What Is a Normal TSH Level?

And What Does the Reference Range Have to Do With It?

Front view of test tubes containing blood samples
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As of late 2016, at most laboratories in the U.S., the official reference range for the Thyroid Stimulating Hormone (TSH) blood test runs from approximately 0.5 to 4.5/5.0. 

The reference range -- also sometimes called the "normal range" -- is an important part of the complete picture of thyroid diagnosis and treatment. In fact,the entire TSH test as diagnostic and management tool depends on this validity.

That is because the reference range is what determines -- for the majority of physicians -- whether or not a thyroid disease or condition is even diagnosed at all, much less treated, and when diagnosed, how it is treated.

A TSH reference range is obtained by taking a large group of people in the population, measuring their TSH levels, and calculating a range that represents healthy levels in that population. Levels below the bottom of the range can then represent hyperthyroidism, 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 made the range less accurate, particularly at the higher end..

This understanding led to a 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 more than decade since the experts recommended this new normal range, doctors have changed their positions yet again, and 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. And laboratories continue to use the older reference ranges for the TSH test. 

In the meantime, patients can seek out 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

Source: Fatourechi V, Klee GG, Grebe SK, et al. Effects of reducing the upper limit of normal TSH values. JAMA. 2003;290:3195-3196.

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