We evaluated plasma selenium concentrations in patients with euthyroid sick syndrome express by low serum triiodothyronine (T3) concentrations. Selenium status in these patients was compared with that found in patients with both untreated and treated hypothyroidism. Selenium concentrations in plasma were significantly lower in hospitalized patients with either euthyroid sick syndrome (0.99 +/- 0.37 mumol/L) or treated hypothyroidism (1.09 +/- 0.25 mumol/L) than in patients with untreated hypothyroidism (1.39 +/- 0.28 mumol/L). However, there was no significant, independent relationship between selenium and the thyroid function indices determined in this study. The strongest association was between serum albumin concentration and either selenium (r = 0.65), T3 (r = 0.58), or the molar ratio of T3 to thyroxine (r = 0.64). The decreased average selenium concentration appears to be associated with the hypercatabolic state of severely ill patients, as indicated by the serum albumin concentration; these patients should be considered for selenium supplementation and their selenium status should be monitored.