Abstract
<title>Abstract</title> <p> Universal salt iodization has substantially reshaped iodine nutrition status in the population. However, current reference intervals (RIs) for thyroid hormones may not be suitable for the elderly. This study aimed to establish appropriate RIs for thyroid hormones and to investigate the association of obesity with subclinical hypothyroidism (SCH) in a Chinese elderly population. This cross-sectional study included 1,404 participants aged ≥ 60 years from iodine-sufficient areas in Tianjin, China. Based on National Academy of Clinical Biochemistry criteria, 539 healthy elderly individuals were selected to establish sex-specific RIs for thyroid hormones using non-parametric methods.The prevalence of thyroid disorders was compared using manufacturer-provided versus newly established RIs. Multivariate logistic and linear regression models were used to examine the associations of obesity and thyroid hormone levels. Sex-specific RIs were established. Applying these RIs significantly reduced the prevalence of SCH from 9.97% to 3.28% ( <italic>P</italic> < 0.001). Multivariate analysis showed that UIC ≥ 300 µg/L( <italic>OR</italic> = 1.84, 95% <italic>CI</italic> : 1.10–3.09), central obesity ( <italic>OR</italic> = 1.80, 95% <italic>CI</italic> : 1.18–2.73), and TPOAb positivity ( <italic>OR</italic> = 2.64, 95% <italic>CI</italic> : 1.61–4.32) were independent risk factors for SCH. In the euthyroid population, WC was positively associated with FT <sub>3</sub> ( <italic>β</italic> = 0.0051, <italic>P</italic> = 0.001) and log10-transformed TSH ( <italic>β</italic> = 0.0011, <italic>P</italic> = 0.046), and negatively associated with FT <sub>4</sub> ( <italic>β</italic> = -0.0269, <italic>P</italic> < 0.001). The newly established sex-specific thyroid hormone RIs for the Chinese elderly population significantly reduced the overdiagnosis of SCH. In addition,central obesity is associated with SCH. </p>