Background and Aims: Correctly characterizing malnutrition is a challenge. Transthyretin (TTR) rapidly responds to adequate protein intake/infusion, which could be used as a marker to identify malnutrition. Nutritional therapy is used to prevent malnutrition. Parenteral nutrition (PN) requires daily monitoring to determine whether what is being offered is adequate. This article aims to investigate whether the practice of measuring TTR is justified. Methods: Data from patients admitted to the ward or intensive care unit (ICU) were collected at three different times: within the first 72 h (T1) of PN use, on the 7th day (T2), and the 14th day (T3) after the initial assessment. Results: 302 patients were included; the average age was 48.3 years old; the prevalence of death was 22.2%, and 61.6% of the sample were male. TTR values and the effectiveness of nutritional support in these patients were not associated with the outcome; however, meeting caloric needs was related to the outcome (p = 0.047). No association was found when TTR values were compared to the nutritional status. Thus, TTR was not a good indicator of nutritional risk or nutritional status in hospitalized patients. Conclusions: Undoubtedly, the TTR measurement was inversely proportional to CRP measurements. It was possible to conclude in this follow-up cohort of hospitalized patients that TTR values were not useful for determining whether the patient was malnourished, predicting death or effectiveness of nutritional support, yet based upon our analyses, a decrease in TTR greater than 0.024 units for every 1 unit increase in CRP might be due to ineffective nutritional supply.