Characterizing the Association Between Blood Urea Nitrogen Levels within the Normal Range and Thyroid Function Among US Adults: NHANES 2007-2012 Findings.

Blood urea nitrogen (BUN) is a key metric when evaluating the renal function of a given patient. Individuals with chronic kidney disease also often present with abnormal thyroid functionality. The specific association between BUN and thyroid function, however, is not well understood. This study was thus developed to explore how BUN levels within the normal range are related to thyroid activity among adults in the USA. The National Health and Nutrition Examination Survey (NHANES) dataset was leveraged for these analyses. Linear regression and smoothed curve fitting approaches were employed to examine the association between BUN levels and measures of thyroid function, with appropriate weighted and subgroup analyses. The 2007-2012 NHANEs cycles included 6231 eligible adults with BUN levels from 2.5-8.0 mmol/l. Under adjusted models, a higher BUN level was associated with reductions in FT3 (β=-0.01; p=0.047), TT3 (β=-1.08; p=0.003), and TT4 (β=-0.10; p=0.003) levels. In subgroup analyses, BUN levels were respectively found to be negatively correlated with FT3 and TT4 levels in subjects who were inactive and active. These findings are consistent with reduced FT3, TT4, and TT4 levels being correlated with higher BUN levels within the normal range among American adults. In non-physically active subjects, FT3 levels were more closely related to increasing BUN levels, whereas TT4 levels were more closely related to BUN levels among physically active subjects. However, additional research will be vital to clarify the mechanisms that underlie these results.