Twenty brain-dead potential organ donors were studied prospectively to establish thyroid function. Two or three consecutive blood samples were obtained during brain death. Seven times a sample was available before brain death occurred. Free triiodothyronine (FT3) fell in most patients (80%). Very low (less than 1.6 pmol/l) and subnormal levels (between 2 and 3 pmol/l) were found in 65% and 15% of the patients, respectively. Serum reverse total triiodothyronine (rT3) was inversely correlated with FT3. Free thyroxine (FT4) was less often decreased (mean 14.68 +/- 1.42 pmol/l) and 35% of the patients had normal levels. Mean thyroid stimulating hormone (TSH) remained normal (0.71 +/- 0.15 microU/ml). The study of consecutive samples during brain death did not show a constant, progressive decrease in hormonal levels. There is no statistical difference between values observed before and after brain death. No correlation was found between FT3 levels and hemodynamic data or immediate allograft function. The pattern of thyroid function in these patients was typical of the sick euthyroid syndrome with a low T3 or low T3 and low T4 serum levels. This syndrome usually does not need to be treated. However, many experiment findings and some clinical data argue in favor of T3 therapy in donors and possibly in recipients. The dosage regimen must be adjusted to be effective without causing harm to multiorgan donors before it can be widely used. It remains to be proved that low FT3 serum indicates low intracellular FT3 and worse metabolic function in clinical conditions.