Relationship between plasma atherogenic index and thyroid function: evidence from NHANES 2007–2012

Thyroid function levels have been associated with a variety of metabolic disorders and correlate with blood lipid levels in vivo. However, no studies have been conducted on the correlation between plasma atherogenic index (AIP) and thyroid function levels. The objective of this study was to explore the relationship between AIP and thyroid function through the National Health and Nutrition Examination Survey (NHANES) database. This study included 8365 non-pregnant adults with complete AIP metric data and thyroid function metric data from the 2007-2012 NHANES database. AIP was calculated as log10 (triglycerides/HDL cholesterol). According to AIP quartile levels, participants were divided into four groups (Q1-Q4). We used weighted linear regression models and restricted cubic spline plots to assess the relationship between AIP and thyroid function levels in conjunction with R software. Besides, subgroup analyses were conducted. In the fully adjusted continuous model, the ORs and 95% CI for TSH, FT3, FT4, TT3, and TT4 were 0.22 (0.10-0.34, P = 0.001), 0.10 (0.07-0.12, P < 0.0001), - 0.04 (- 0.06 to 0.03), P < 0.0001), 5.32 (2.86-7.78, P < 0.001), and 0.09 (- 0.05 to 0.22, P = 0.19). This correlation remained in the fully adjusted classification model. No significant interaction between subgroups was shown in the subgroup analyses. This study demonstrates that higher AIP levels in US adults are linearly positively correlated with TSH, linearly negatively correlated with FT4, nonlinearly positively correlated with FT3, and "inverted U-shaped" nonlinearly correlated with TT3 and TT4.