Biological variation in thyroid function tests in older adults and clinical implications

ObjectiveInterpreting thyroid function tests can be challenging due to inherent variation, and the need for tests rises with age. While age-related changes in thyrotropin (TSH) levels are known, the biological variation in older adults remains unclear.DesignWe recruited nineteen 65-99-year-old (older adults) without thyroid disease for monthly blood sampling for 1 year.Patients and MeasurementsSerum was stored at -20C degrees, and TSH, total thyroxine (TT4) and total triiodothyronine (TT3) were analysed in random order in a single batch for each participant. Results were compared to test results from 15 euthyroid men aged 24-53 years (younger adults) collected previously using a similar methodology.ResultsInterindividual coefficients of variation in older/younger adults were 46.7%/44.0% for TSH, 12.7%/19.5% for TT4 and 14.6%/22.4% for TT3. Intraindividual coefficients of variation (CVI) were 19.0%/25.4% for TSH, 5.5%/10.8% for TT4 and 6.9%/13.2% for TT3. The index of individuality was below 0.6 for all hormones in all age groups. The number of samples required to determine the homoeostatic set-point at 10% precision in older adults was 14-21 for TSH and 2 for TT4 and TT3. TT4 in older adults was the only parameter in any group with comparable CVI between individuals (p = .22).ConclusionsCVI for TT4 and TT3 was halved in older compared to younger adults with two tests of TT4 needed to describe the individual set-point. Similar CVI between older adults caused TT4 to provide a reliable estimate of thyroid function, and the added value of measuring thyroxine could improve clinical practice.

Automated summary

This study investigated the variation in thyroid function test results in older adults and younger adults. The results showed that the coefficients of variation for TSH, TT4, and TT3 were higher in older adults compared to younger adults. However, TT4 had comparable coefficients of variation between individuals in older adults, indicating that it could provide a reliable estimate of thyroid function.