1. Subjects followed serially after acute myocardial infarction demonstrated a rapid and sustained fall in serum total tri-iodothyronine (T3) concentration and a rise in reverse tri-iodothyronine (rT3) concentration. There was a transient fall in total thyroxine (T4) concentration.
2. Thyroxine binding globulin (TBG) levels were unchanged after acute myocardial infarction but prolonged falls were observed in thyroxine binding prealbumin (TBPA) and albumin concentrations.
3. In contrast to the fall in total T4, both measured and calculated free T4 concentrations were unchanged but measured and calculated free T3 concentrations fell as did total T3.
4. Despite the observed fall in T3, basal thyrotrophin (TSH) concentrations did not rise.
5. The reduction in circulating T3 levels after acute myocardial infarction suggests that a hypothyroid state exists. Until tissue thyroid status can be assessed directly, however, this conclusion must remain in doubt.