The free tetraiodothyronine index (FT4I) and free triiodothyronine index (FT3I) in burn patients represented the serum levels of free (dialyzable) T4and free T3, respectively. FT4I and FT3I were lower with greater burn size and were lower in nonsurvivors than expected for the burn size. There was no compensatory elevation of basal or releasing hormone-stimulated thyrotrophin (TSH) concentrations. Reverse T3was higher with greater burn size. T3treatment restored FT3I but did not affect mortality or resting metabolic rate (MR) measured in survivors, compared with placebo therapy. Whereas the hypermetabolic response to burn injury appeared to be independent of thyroid hormones, MR was correlated positively with burn size and with elevated plasma nor-epinephrine and epinephrine concentrations for several weeks after injury. Lack of augmented TSH concentrations, absence of low plasma reverse T3, and presence of hypermetabolism suggest that the reduced plasma free T3does not indicate functional hypothyroidism, but may represent an adaptation to the assumption of metabolic control by the sympathetic nervous system.