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Some endocrinologists have proposed that the upper limit of normal (ULN) for thyrotropin (TSH) be lowered to 2.5 μIU/mL (from the traditional ULN of 4–5 µIU/mL). This controversial proposal is related to uncertainty about the prevalence of early subclinical hypothyroidism in “normal” populations, from which the reference ranges are derived.
In this study, 1861 randomly selected middle-age adults from eastern Washington State underwent TSH and thyroid antibody testing as well as clinical and ultrasound examination of their thyroids. The 97.5 percentile of TSH (the standard cutoff for determining the ULN) was as follows:
5.67 µIU/mL in the entire cohort
3.73 µIU/mL after excluding 1448 people who reported histories of thyroid disease or whose th…