Reduced Serum T4and T3and Their Altered Serum Binding after Burn Injury in Rats

Total T4and T3concentrations are often suppressed in burned patients. To investigate the significance of such changes, we have characterized serum T4and T3after full-thickness scald burns (60 body surface under anesthesia) of 270-gm male Sprague-Dawley rats housed in a light:dark cycle of 14:10 hr. Groups (N= 9–15) of BURN, SHAM (anesthesia, fur clipped, no burn) and CON (controls) were sacrificed on postburn days 8 and 14. T4and T3(radioimmunoassay), free indices (FT4I and FT3I = respective total T4or T3× in vitro charcoal T3uptake, T3U), and free concentrations (FT4and FT3= total T4or T3× respective equilibrium dialyzable fraction, T4DF or T3DF) were not different between CON and SHAM. Compared to SHAM, mean T4and FT4I (by about 48 of respective SHAM means on both days), TT3(by 36, 43), and FT3I (by 38, 45) (days 8, 14) were suppressed in BURN (all p< 0.001). T4DF (both days) and T3DF (day 14) were significantly elevated in BURN, demonstrating a deficit in serum binding, but T3U was not. FT4(by 26, 22) and FT3(by 33, 34) (day 8, 14) were significantly lower in BURN. On either day, covariance analyses (BURN vs. combined CON+SHAM) correlated FT4I or FT3I with respective FT4or FT3(all p< 0.001, slopes not different in BURN vs. CON+SHAM), but the lower FT4I and FT3I in BURN significantly overestimated (all p< 0.001) the depression of respective FT4and FT3in BURN. Similarly to patients, burned rats exhibit suppressed circulating total and free T4and T3concentrations despite elevated dialyzable (free) fractions of T4and T3. Because of failure of the T3U to account for this serum binding abnormality, the results are most consistent with a burn-induced circulating inhibitor(s) for binding of T4and T3, not only to transport proteins but also in vitro charcoal, perhaps similar to inhibitors previously described in the sera of patients with various nonthyroidal illnesses. The thermally traumatized rat appears to be a good model for thyroid changes in burns and other nonthyroidal illness.