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Background: Subclinical and overt thyroid dysfunction is easily detectable, often modifiable, and, in younger age groups, has been associated with clinically relevant outcomes. Robust associations in very old persons, however, are currently lacking. This study aimed to investigate the associations between (sub-)clinical thyroid dysfunction and disability in daily living, cognitive function, depressive symptoms, physical function, and mortality in people aged 80 years and older. Methods: Four prospective cohorts participating in the Towards Understanding Longitudinal International older People Studies (TULIPS) consortium were included. We performed a two-step individual participant data meta-analysis on source data from community-dwelling participants aged 80 years and older from the Netherlands, New Zealand, United Kingdom, and Japan. Outcome measures included disability in daily living (disability in activities of daily living [ADL] questionnaires), cognitive function (Mini-Mental State Examination [MMSE]), depressive symptoms (Geriatric Depression Scale [GDS]), physical function (grip strength) at baseline and after 5 years of follow-up, and all-cause five-year mortality. Results: Of the total 2116 participants at baseline (mean age 87 years, range 80–109 years), 105 participants (5.0%) were overtly hypothyroid, 136 (6.4%) subclinically hypothyroid, 1811 (85.6%) euthyroid, 60 (2.8%) subclinically hyperthyroid, and 4 (0.2%) overtly hyperthyroid. Participants with thyroid dysfunction at baseline had nonsignificantly different ADL scores compared with euthyroid participants at baseline and had similar MMSE scores, GDS scores, and grip strength. There was no difference in the change of any of these functional measures in participants with thyroid dysfunction during five years of follow-up. Compared with the euthyroid participants, no 5-year survival differences were identified in participants with overt hypothyroidism (hazard ratio [HR] 1.0, 95% confidence interval [CI 0.6–1.6]), subclinical hypothyroidism (HR 0.9 [CI 0.7–1.2]), subclinical hyperthyroidism (HR 1.1 [CI 0.8–1.7]), and overt hyperthyroidism (HR 1.5 [CI 0.4–5.9]). Results did not differ after excluding participants using thyroid-influencing medication. Conclusions: In community-dwelling people aged 80 years and older, (sub-)clinical thyroid dysfunction was not associated with functional outcomes or mortality and may therefore be of limited clinical significance. [ABSTRACT FROM AUTHOR]