The Thyrotropin Reference Range Should Be Changed in Older Patients

摘要

Subclinical hypothyroidism is a laboratory test–defined condition that involves a thyrotropin level above the reference range concomitant with free thyroxine levels within the reference range.1 The current thyrotropin reference range is based on the 95% CI of the distribution of levels in a population without known thyroid disease; individuals with thyrotropin values in the lowest and highest 2.5% of this distribution are outside the range, and these values are considered abnormal. Despite the therapeutic implications, the upper and lower limits of the thyrotropin reference range are not derived from data demonstrating benefit from treatment at these thresholds. Furthermore, the distribution of thyrotropin levels in the population differs by age, with a shift toward higher levels with increasing age.2 There are 2 implications from this finding. First, the shape of the thyrotropin distribution suggests an age-related population shift rather than increased incidence of hypothyroidism at older ages. Second, subclinical hypothyroidism is more likely to be detected in older individuals, and is present in 14.5% of individuals aged 80 years and older, compared with the expected 2.5% of individuals aged 20 to 29 years.2 Levothyroxine is a medication with a narrow therapeutic window, and overreplacement is common in older individuals.3 Understanding the consequences of a diagnosis of subclinical hypothyroidism, including the risks and benefits of levothyroxine treatment, is particularly important in older patients.