Variations in serum T3, rT3, 3,3'-diiodothyronine and 3',5'-diiodothyronine induced by acute myocardial infarction and propranolol

Abstract

Abstract. Serum levels of thyroxine, 3,5,3'-triiodothyronine (T 3 ), 3,3',5'-triiodothyronine (rT 3 ), 3,3'-diiodothyronine (3,3'-T 2 ), 3',5'-diiodothyronine (3',5'-T 2 ) and TSH were measured in two clinical situations which are both known to induce a low serum T 3 high serum rT 3 syndrome: 1) during the early course of acute myocardial infarction (AMI) and after recovery, and 2) before and during one week's propranolol medication (20 mg 4 times a day). In 10 patients with AMI serum levels of the iodothyronines were unchanged on admission to hospital (in average 6.6 h after onset of symptoms). However, already 24 h after onset of symptoms serum T 3 and 3,3'T 2 were reduced whereas serum rT 3 and 3',5'-T 2 were increased. Serum T 3 and 3,3'-T 2 reached a nadir on day 4 and 3, respectively, whereas serum rT 3 and 3',5'-T 2 reached peak values 24 h after onset of symptoms. In eight healthy, euthyroid volunteers propranolol medication induced similar changes in iodothyronine concentration as AMI did. However, the alterations were more delayed. Serum T 3 decreased slowly reaching statistically significantly reduced values on day 7. Serum rT 3 and 3',5'-T 2 were significantly enhanced from day 3 and 4, respectively. A close parallelism in alterations of serum T 3 and 3,3'-T 2 levels was observed. Our data suggest that T 3 in the two situations studied is a major precursor for 3,3'-T 2 probably as a consequence of reduced 5'-deiodinase activity. It seems possible that the mechanisms affecting the metabolism of the iodothyronines in AMI and during propranolol medication involved the same enzyme system. However, the late appearance of the alterations in serum iodothyronines levels during propranolol medication might indicate different modes of action.