To investigate the relationship between serum T3 levels and thermogenesis in euthyroid subjects, the conversion of T4 to T3 was partially blocked over a period of 3 weeks by weekly ingestion of 2 g iopanoic acid. Serum T3 decreased by 56 +/- 6 ng/100 ml without affecting the metabolic rate or total heat loss. Because an increase in serum T4 levels was observed (3.70 +/- 0.2 microgram/100 ml), the experiment was repeated in T4-substituted subjects. Serum T3 decreased even more in these subjects, (by 85 +/- 12 ng/100 ml), and the increase of serum T4 was also larger (4.6 +/- 0.7 microgram/100 ml). Again, the metabolic rate did not change. Experiments performed on hypothyroid patients and T3-substituted subjects demonstrated that iopanoic acid alone was without effect on the metabolic rate. Furthermore, the effect of a low energy, high protein diet (1672 kilojoules or 400 Cal) on the metabolic rate was confirmed. These results show that the decrease in magnitude of serum T3 seen with iopanoic acid is not sufficient by itself to reduce the metabolic rate.