The effect of complete fasting on the serum concentrations of the iodothyronines 3,5-diiodothyronine (3,5-T2), 3,3'T2, 3', 5'-T2 and 3'-monoiodothyronine (3'-T1) was evaluated. Fourteen obese women underwent a complete fasting for 4 days. Caloric restriction resulted in the following serum hormone levels (before vs 3. day): T4: 103 vs 109 nmol/l (NS), T3: 1.83 vs 1.24 nmol/l (p less than 0.01), rT3: 0.276 vs 0.407 nmol/l (P less than 0.01), 3.5-T2: 70 pmol/l (NS), 3.3'-T2: 42 vs 39 pmol/l (p less than 0.01), 3',5'-T2: 63 vs 93 pmol/l (P less than 0.01), and 3'-T1 60 vs 116 pmol/l (P less than 0.01). All subjects were refed with 200 g (800 kcal, 3350 kJ) d-glucose per day in divided doses for 2 days. Refeeding tended to normalize the changed iodothyronine concentrations and there was no difference whether the glucose was administered by the oral (n = 7) or the intravenous route. In can be concluded that starvation in man is accompanied by profound changes in peripheral metabolism of the T2's and 3'T1. There seems to be no qualitative difference of the effect on the thyroid hormone metabolism of d-glucose administered by the oral or the intravenous route.