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

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Abstract

Objective: Interpreting 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.

Design: We recruited nineteen 65-99-year-old (older adults) without thyroid disease for monthly blood sampling for 1 year.

Patients and measurements: Serum was stored at -20C°, 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.

Results: Interindividual 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).

Conclusions: CVI 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.
Original languageEnglish
JournalClinical Endocrinology
Volume99
Issue number6
Pages (from-to)598-605
Number of pages8
ISSN0300-0664
DOIs
Publication statusPublished - Dec 2023

Keywords

  • biological variation
  • coefficient of variation
  • intraindividual variation
  • older adults
  • thyroid function

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