In patients with severely acute diseases, a special relationship of thyroidal hormones with decreased T3 and increased rT3 levels is known, the so-called low T3 syndrome. The aim of this study was to elucidate the involvement of the hypothalamo-pituitary thyroid axis, the pituitary-gonadal axis, the altered hepatic function, the plasma proteins in the low T3 syndrome, and the evaluation of these parameters for prognosis in patients with acute myocardial infarction. Thirty-one patients (29 male, 2 female) with AMI entered the study for the determination of hypothalamo-pituitary thyroid axis and the plasma proteins. Besides routine laboratory determinations, TRH, TSH, T4, T3, rT3, CHE, albumin, total protein, TBG, and estradiol concentrations in plasma were measured daily for 5 days after AMI using immunological and other methods. Twelve male patients with AMI entered the study for the determination of pituitary-gonadal axis; the T3, rT3, estradiol, testosterone, FSH, and LH concentrations in serum were determined using immunological methods. We found that T3 and T4 decreased significantly to a minimum on the first and the second day, respectively, after admission and increased in the course of the observation period. In contrast, rT3 was elevated significantly within the first 2 days and decreased later. TSH and TRH decreased in the first 2 days and increased in the following days. CHE, albumin, and total protein levels significantly showed a minimum on day 4 and TBG significantly showed a minimum on the second day after AMI and increased to day 4.(ABSTRACT TRUNCATED AT 250 WORDS)