Thyroid Function and Reduced Kidney Function with Aging in Chinese Older Adults: A Cross- sectional Study

Background: Thyroid function may be a factor influencing renal function in general population. Renal function and thyroid function vary with aging, thus the association of thyroid function and declining kidney function in older adults was different from this in the young and remains controversial. The aim of this study was to estimate association between thyroid function in reference interval and reduced kidney function with aging in Chinese older adults. Methods: A total of 15729 adults, 23.3% (N= 3624) of which were older population, were collected in the health check-up department of the First Affiliated Hospital of Nanjing Medical University between January 2018 to January 2020.Basic demographic information was collected by physician-administered questionnaire. The variation tendency of thyroid function with aging was investigated by mean of free triiodothyronine (FT3), free thyroxine (FT4) and thyroid stimulatinghormone (TSH) in subgroups for every 10 years. Associations between kidney and thyroid function were estimated with β value by multiple linear regression and with odds ratio (OR) by multivariable logistic regression models adjusted for age, gender, body mass index and serum urine acid. Results: In the older population, an increased trend of TSH, a decreased trend of FT3 and little change of FT4 were identified with aging. Estimated glomerular filtration rate (eGFR) remarkably decreased with increasing TSH (β: -0.081) and declining FT3 (β: 0.083) concentrations. Compared with the population in the lowest FT3 quartile (3.10-4.45 pmol/L), the prevalence of eGFR< 75ml/min/1.73m 2 significantly decreased by 22.0% with FT3 4.46-4.81 pmol/L, 27.6% with FT3 4.82-5.20 pmol/L and 34.9% with FT3 5.21-6.8 pmol/L in older individuals ( P for trend <.001). The OR was 1.315 with high-normal TSH, comparing to low-normal TSH in older persons ( P 0.025). Similar results were found between prevalent eGFR< 60ml/min/1.73m 2 and thyroid function. The prevalent reduced kidney function was not remarkably associated with FT4 in the reference range. Conclusions: This study demonstrated a significant association between thyroid function and kidney function, especially FT3 in older population. The clinician were advised to evaluate renal function with low-normal FT3 and high-normal TSH in case of potential decreased kidney function.