Physical performance across the thyroid function values within the normal range in adult and older persons

Background: Thyroid hormone variation may be correlated with adverse health outcomes, even within the normal reference range in euthyroid individuals. Aims: To determine the association between plasma thyroid-stimulating hormone (TSH), free triiodothyronine (FT3), and free thyroxine (FT4) levels and physical performance score in middle age and older adults who had levels of all three hormones in the normal range. Methods: In this community-based, cross-sectional study, euthyroid participants of the Invecchiare in Chianti study, aged 23–102 years (N = 1060), were considered. Physical performance was evaluated by the Summary Physical Performance Battery (SPPB) score. Plasma TSH, FT3, and FT4 levels were predictors, and SPPB score was the outcome. Results: At the univariate analyses, TSH, FT4, and FT3 were not significantly associated with SPPB score in young individuals, whereas, in older participants, SBBP score was positively (P < 0.001) associated with FT3, and negatively associated with both TSH (P < 0.02) and FT4 (P < 0.001). After adjusting for multiple confounders, FT3 remained significantly associated with SPPB (beta ± SE, 0.35 ± 0.17, P = 0.04), but FT4 and TSH were not. Results did not change when all the three hormones FT3, FT4, and TSH were simultaneously considered in the fully adjusted model (beta ± SE for FT3, 0.37 ± 0.18, P = 0.04). Discussion: The results of this study demonstrate that SPPB score is positively associated with circulating FT3 but not with FT4 or with TSH, in older euthyroid individuals. Conclusions: In euthyroid older adults, circulating FT3 may play an important role in the thyroid effects on physical function.