Thyroid Testing: What are Normal TSH Levels?

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 2017, 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 or 5.0. 

The reference range—also sometimes called the "normal range"—is an important part of your overall thyroid diagnosis and treatment program, because the conventional endocrinology world views it as the "gold standard" measurement of your thyroid function.

A patient whose TSH level is within the reference range is referred to as "euthyroid." 

There is a challenge, however, in reliance on the TSH test as a primary diagnostic and management tool: a disagreement among practitioners over what depends on this validity. When the TSH reference range is used by the majority of physicians to determine whether or you have thyroid disease, or whether it is being adequately treated, it is important to know that different practitioners are following different guidelines in interpreting the meaning of test results. 

How is the TSH Reference Range Determined?

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 is supposed to represent the range of TSH levels in a healthy population. Levels below the bottom of the range then suggest hyperthyroidism, and levels above the top end of the range suggest hypothyroidism.

What some experts have identified, however, is that TSH reference range includes people who actually have already developed a mild or subclinical form of thyroid disease. The reference range also includes those who have elevated antibodies indicative of Hashimoto's disease, but whose TSH is not yet elevated to reflect progressing hypothyroidism.

This means that some people with TSH levels at the higher end of the range have a thyroid condition, yet their levels are being included when developing reference ranges meant to represent a healthy population without thyroid problems. 

The TSH Reference Range Controversies

There is a controversy within the conventional endocrinology community, as well as with integrative practitioners. In 2003, the American Association of Clinical Endocrinologists recommended 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. The group had reported:

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 was recommended by AACE and the National Academy of Clinical Biochemistry, what were 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 been more than decade since the experts recommended this new normal range. In that time, the endocrinology authorities again changed their position, and abandoned their official recommendation to narrow the range. Laboratories continue to use the older reference ranges for the TSH test. The majority of conventional endocrinologists and physicians are using the current reference range, and view returning patients to a euthyroid status as full treatment.

Some, however, are using a narrower range—or adding antibodies tests—for diagnosis and therapeutic management of their thyroid patients. 

At the same time, integrative physicians and hormone experts have increasingly argued for "optimal" treatment, versus using the broad reference range to diagnose and manage thyroid treatment. 

Your Next Steps?

If you have a TSH level that is in the higher end of the reference range, you may need to find a practitioner who uses a narrower reference range for diagnosis or management. Another option is to seek out a practitioner who will considers your symptoms and history, will test antibodies and Free T4/Free T3 levels, in making a diagnosis or prescribing and management of your treatment. You may also want to learn more about in an interview with a prominent endocrinologist, Dr. Jeffrey Garber: "TSH Normal Range: Why is There Still Controversy? 

Sources:

Braverman, L, Cooper D. Werner & Ingbar's The Thyroid, 10th Edition. WLL/Wolters Kluwer; 2012.

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

Garber, J, Cobin, R, Gharib, H, et. al. "Clinical Practice Guidelines for Hypothyroidism in Adults: Cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association." Endocrine Practice. Vol 18 No. 6 November/December 2012.

Gursoy A, et. al. "Which thyroid-stimulating hormone level should be sought in hypothyroid patients under L-thyroxine replacement therapy?" Int J Clin Pract. 2006 Jun;60(6):655-9​

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