We examined the changes in thyroid hormone levels in patients with an acute clinical condition and compared these to levels in the healthy subjects. Serum total triiodothyronine (T3), thyroid stimulating hormone (TSH), and free thyroxine (fT4) measurements were recorded from 555 patients (mean age: 55.0 years, men: 65.9%) admitted to the emergency department (ED) 1-91 months (median: 34 months) after a regular health examination (HE). Serological data were analyzed; mean change in hormone levels was stratified by emergency classification system and quintiles of changes in inflammatory marker values, such as neutrophil lymphocyte ratio (NLR) and high-sensitivity C-reactive protein (CRP). The mean decrease in T3 levels from HE and ED samples was 10.6 ng/dL (p< 0.001). Mean decrease in T3 levels was 21.6 ng/dL among patients classified as having an infection status and 11.0 ng/dL among patients classified as having an urgency status. A decrease 3.7 ng/dL among emergency patients was observed. TSH and fT4 levels did not change across all groups. When patients were stratified into quintiles according to changes in NLR values, mean decreases in T3 were 6.21, 8.14, 14.37, 12.76, and 21.98 ng/dL and showed significant linear reduction (p<0.001). For quintiles of changed CRP values, mean decreased T3 levels were 10.57, 3.05, 4.47, 7.68, and 28.07 ng/dL. TSH and fT4 were not associated with significant changes (p = 0.100, p = 0.561, respectively). In this study, thyroid function changes in individuals with an acute condition revealed that T3 significantly decreased, more markedly in infectious diseases compared to their healthy counterparts, and decline in T3 measurements correlated with inflammatory markers. TSH and fT4 levels remained stable. It is necessary to consider the severity of acute conditions when abnormal T3 levels are detected in subjects with emergent status.