Background: Hormone level changes after heart surgeries are a widely observed phenomenon due to neurohormonal feedback mechanisms that may affect postoperative morbidity and mortality. The current study aimed to analyze the changes in thyroid and sex hormones in the first 24 postoperative hours after heart surgery. Methods: This prospective, observational study (registered on ClinicalTrials.gov: NCT03736499; 09/11/2018) included 49 patients who underwent elective cardiac surgical procedures at a tertiary heart center between March 2019 and December 2019. Thyroid hormones, including thyroidstimulating hormone (TSH), triiodothyronine (T3), and thyroxine (T4), and sex hormones, including prolactin (PRL) and total testosterone, were measured preoperatively and at 24 h postoperatively. Results: Significant decreases in serum TSH (P < 0.001), T3 (P < 0.001) and total testosterone (P < 0.001) levels were noted, whereas T4 (P 5 0.554) and PRL (P 5 0.616) did not significantly change. Intensive care unit (ICU) hours (P < 0.001), mechanical ventilation (P < 0.001) and Vasoactive-Inotropic Score (VIS) (P 5 0.006) were associated with postoperative T3 level. ICU hours were associated with postoperative T4 level (P 5 0.028). Postoperative and delta testosterone levels were in connection with lengths of stay in ICU (P 5 0.032, P 5 0.010 respectively). Model for End-Stage Liver Disease (MELD) scores were associated with thyroid hormone levels and serum testosterone. Conclusions: T3 may represent a marker of nonthyroidal illness syndrome and testosterone may reflect hepatic dysfunction. In addition, PRL may act as a stress hormone in female patients.  Kulcsszavak: cardiac surgical procedures, thyroid function, nonthyroidal illness, prolactin, testosterone