In severe illness of any cause, downregulation of the thyroid hormone system may occur. How this affects patients with acute myocardial infarction (AMI) is largely unknown. To investigate changes in serum levels of the thyroid hormones during AMI in the CAD-prone South Indian population. In this study, thyroid parameters were estimated in a series of 30 consecutive infarction patients treated by thrombolysis. Blood samples were taken at time of admission before therapy (4h), after 12h, 24h, 36h, 48h and 7th day following admission and we investigated the possible relationship of these alterations with the severity of AMI. Fifteen patients had left ventricular ejection fraction (LVEF) < 50% (group I) and 15 patients had LVEF > 50% (group II). A transient decrease in total tri-iodothyronine (T3) occurred at 24 hours after onset of symptoms, in both groups. Total thyroxine and TSH levels did not change significantly. Negative correlation was found between total T3 and CK-MB levels (p < 0.001). The thyroid hormone system is rapidly down regulated in AMI. This may be beneficial during acute ischaemia. The degree of T3 decrease is proportional to the severity of cardiac damage and may have a possible prognostic value.