Free T₄, free T₃, and reverse T₃in critically III, thermally injured patients

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 T₃values were significantly lower in the unstable patients (p < 0.01). All FT₃values for the unstable patients (M ± SE), 193 ± 14 pg/dl, were below the normal range for FT₃of 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 hypothyroidism with clinical deterioration may have functional significance for the critically ill trauma patient.