The Risks of High-Normal and Low-Normal TSH Thyroid Levels

Subclinical Hypothyroidism and Subclinical Hyperthyroidism

tsh levels, high-normal TSH, low-normal TSH, tsh reference range
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There is a great deal of controversy in the endocrinology world regarding the health implications of having a thyroid stimulating hormone (TSH) level in the higher end of the reference range, a situation sometimes defined as subclinical hypothyroidism. Similarly, there is also controversy over TSH levels that are in the low end of the range—known as subclinical hyperthyroidism.

Some studies have shown a connection between subclinical and overt hypothyroidism and a number of health conditions, including:

While the research is murkier for the implications of subclinical hyperthyroidism, researchers have shown an increased risk of atrial fibrillation in people with an borderline overactive thyroid gland.

Research published in 2016 looked in great depth at the relationship between TSH levels and the risk of mortality in adults in the United States.

The prospective study evaluated more than 12,000 adults in the United States whose TSH levels fell within the reference range, broken down by different levels within that range. Specifically, they defined the following three categories within the reference range:

  • Low-normal TSH group: TSH of 0.39–1.15
  • Medium-normal TSH group: TSH of 1.16–1.89
  • High–normal TSH group: TSH of 1.90–4.60

    The researchers found a significantly higher risk of mortality from a variety of causes—including heart disease and cancers such as liver cancer and post-menopausal breast cancer—in those who had high-normal or low-normal TSH levels, as compared to those who had medium-normal TSH levels.

    This study appeared to be the first that showed a link between high- and low-normal TSH levels and an increased risk of death.

    According to the researchers, these findings raise the issue of reconsidering the reference range for the TSH test. The reference range—sometimes called the “normal range”—is determined by evaluating the TSH levels of a large population, and calculating a range that represents healthy thyroid function in that population.

    The researchers indicated that these findings suggest that the reference range level for TSH tests should be reevaluated, and that thyroid experts should consider a potentially narrower range that better reflects the TSH levels associated with better health and reduced mortality risk.

    What Does This Mean for Patients?

    The issue of the TSH reference range has been controversial for decades.

    In 2002, the American Association of Clinical Endocrinologists (AACE) recommended a narrowing of the TSH reference range, bringing down the top end of the range 3.0. The AACE decision to narrow the range was a result of data that suggested that many people who had low-level thyroid problems—reflected by levels within the old reference range—could be improved with treatment.

    At that time, the AACE president, Hossein Gharib, MD, FACE, 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."

    That narrower range doubled the number of people in the United States who could be diagnosed with abnormal thyroid function, and for a brief time, thyroid disease outpaced diabetes as the most common endocrine disorder in North America.

    The recommended new guidelines were debated, and ultimately never widely adopted, much less incorporated into official treatment guidelines, or laboratory testing reports. In the end, the impact of the AACE’s recommendation was severely blunted. Several years later, the AACE quietly dropped the recommendation, scrubbed its website of the initial announcements of the recommended changes to the range, and, quite frankly, went on as if the whole episode had never happened.

    Since that time, the battle over the TSH reference range has continued, with some experts staking out the position that the reference range should not change, and others arguing that a narrower range would better protect the health of the population.

    Ten years later, the controversial clinical guidelines for hypothyroidism did not address the controversy over the reference range.

    In the end, some experts agree that the TSH test is just one parameter among many, including Free T4 and Free T3 levels, as well as symptoms, to evaluate and address as part of optimal thyroid health. As a patient, if you have doctors who are determined to keep you on the edges of the reference range and you don’t feel well, it may be time to find a new practitioner for your thyroid care.

    Source:

    Kosuke, Inoue, et. al. “Association Between Serum Thyrotropin Levels and Mortality Among Euthyroid Adults in the United States” Thyroid. October 2016, 26(10): 1457-1465. doi:10.1089/thy.2016.0156.

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