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December 24/31, 2003

Effects of Reducing the Upper Limit of Normal TSH Values

Author Affiliations

Letters Section Editor: Stephen J. Lurie, MD, PhD, Senior Editor.

JAMA. 2003;290(24):3195-3196. doi:10.1001/jama.290.24.3195-a

To the Editor: Measurement of the serum thyrotropin (TSH) concentration is the most sensitive single test of thyroid function. Concentrations of TSH in the apparently healthy population are log-normally distributed; 70% to 80% are between 0.3 and 2.0 mIU/L, while 97.5% are less than 5.0 mIU/L.1 When individuals with thyroid autoantibodies, goiter, or a strong family history of thyroid disease are excluded, the upper bound of the 95% TSH concentration reference range decreases to between 2.5 and 3.0 mIU/L.1,2 It has been argued that such a "refined normal range" is a better reflection of thyroid health than a standard population-based reference range and that elevated values may predict future hypothyroidism.15 This decreased upper bound would increase the sensitivity of the diagnosis of subtle hypothyroidism while decreasing its specificity. We investigated the potential impact of the suggested change in reference range.

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