The primary findings of this study are as follows: (1) FT4 was the only index among thyroid hormones that was not correlated with APACHE-II score and still related to the Cysc level after adjusting for several confounding factors; (2) there was no significant difference in the diagnostic performance of Cysc among patients with different FT4 levels; and (3) the optimal cut-off value of Cysc in diagnosing AKI increased as FT4 increased. The gene discussed is TG; the disease is acute kidney injury.