A prospective study of thyroid function was performed in 25 thermally injured patients. These patients are divided into two groups. The first group contains five patients with greater than 50% burn size studied longitudinally during the first 15 days following thermal injury. Significant suppression of serum concentrations of 3,5,3'-triiodothyronine (T3) and elevation of serum concentrations of 3,3',5'-triiodothyronine (rT3) were seen. The free thyroxine index and serum TSH concentrations remained within the normal range. To assess the clinical significance of these alterations in peripheral thyroid hormone concentrations, a second group of 20 patients was studied. We measured the free serum levels of T4 (FT4) and T3 (FT3) in ten patients, mean age 34 years, mean burn size 56%, studied during a period of clinical deterioration, and in ten patients of comparable age and burn size who were clinically stable. Both FT4 and T3 values were significantly lower in the unstable patients (p < 0.01). All FT3 values for the unstable patients (M +/- SE), 193 +/- 14 pg/dl, were below the normal range for FT3 of 230 to 669 pg/dl, and significantly lower than those observed in the stable patients (M +/- SE), 430 +/- 59 pg/dl (p < 0.001). This correlation of biochemical hypothyrodisim with clinical deterioration may have functional significance for the critically ill trauma patient.