Association Between Plasma and Urinary Reduced Thiols in Essential Hypertension: Evidence from a Paired Observational Study

<b>Background/Objectives:</b> Oxidative stress and extracellular redox alterations are involved in the pathophysiology of essential hypertension, but their clinical assessment is limited by the invasiveness and preanalytical complexity of blood-based measurements. Urine represents an attractive non-invasive biological matrix; however, the relationship between urinary and plasma DTNB-reactive reduced thiols in hypertensive patients remains insufficiently characterized. This study aimed to evaluate the association between plasma and urinary reduced thiols in essential hypertension. <b>Methods:</b> In this paired observational study, plasma and urine samples were obtained from 40 patients with treated essential hypertension. Reduced thiols were quantified using a DTNB-based colorimetric assay under identical analytical conditions. Plasma thiols were normalized to total plasma protein concentration, and urinary thiols were normalized to creatinine. Associations between plasma and urinary thiols were assessed using non-parametric correlation analyses. <b>Results:</b> Protein-normalized plasma thiols and creatinine-normalized urinary thiols showed a significant positive correlation (Spearman's ρ ≈ 0.7, <i>p</i> < 0.001). <b>Conclusions:</b> In patients with essential hypertension, creatinine-normalized urinary reduced thiols are strongly associated with protein-normalized plasma reduced thiols, as measured by the DTNB reaction method. These findings provide hypothesis-generating evidence that urinary thiols may reflect extracellular thiol-related redox alterations, warranting further validation in independent and more diverse cohorts.