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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.1-5 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.