Although alterations of serum thyroid hormone indices are frequent in nonthyroidal illnesses, their relationship to survival is poorly defined. Consequently, the prevalence and prognostic relevance of alterations in thyroidal indices were evaluated prospectively in 195 patients requiring intensive medical therapy and in 75 critically ill patients with serum total T4 (TT4) levels below 3 μg/dl. In the 195 patients, serum total T3 (TT3) and TT4 levels were reduced in 69% and 43% respectively. Decreased TT4 levels had the highest correlation with mortality (P < 0·001) and correctly predicted outcome in 70% of patients. Other thyroidal indices, which were significantly different between survivors and nonsurvivors, correlated with TT4 and did not contribute independently to prediction accuracy when assessed by discriminant function analysis.

In seventy‐five patients with TT4 levels below 3 μg/dl, the nadir TT4 level appeared a median of 9 days (range 1–28) after hospital admission. The nonsurvivors lived a median of 6 days (range 0–42) and the survivors were discharged a median of 20 days (range 4–55) after the TT4 nadir. Only an increased free fraction of T4 (FFT4) was found to correlate significantly with mortality and contribute to prediction accuracy.

An inhibition of serum T4 binding to carrier proteins could account for both the decreased serum TT4 and the increased FFT4 values. Serum TT4 and/or FFT4 levels may be useful in assessing the severity and predicting the clinical outcome of acute critical nonthyroidal illnesses.