Diabetes mellitus and fasting are both associated with low plasma thyroid hormone concentrations and loss of body weight. To discriminate between the separate effects of energy shortage and insulin, we studied control rats, diabetic rats (DM), DM rats treated with insulin (DMI), and rats after modified fasting (MF1 and MF2; 70 and 30% of normal daily food intake, respectively). In double-isotopic equilibrium experiments, we determined the tissue thyroxine (T4) and triiodothyronine (T3) concentrations and the contribution of local T4-to-T3 conversion to total T3 in rat tissues; thyroidal T4 and T3 secretion and extrathyroidal T3 production were calculated. In DM and DMI rats, plasma T4 and T3 decreased; in MF1 and MF2 rats, only plasma T4 decreased. Thyroidal T4 secretion decreased, whereas that of T3 remained normal. The decrease in tissue T4 in MF and DM rats paralleled the decrease in plasma T4. Although plasma T3 did not differ in DM and DMI rats, total T3 concentrations in all tissues were not the same due to changed uptake of T3 from plasma and local T4-to-T3 conversion; these changes were not found in several tissues of MF1 and MF2 rats. Our results suggest that the decrease in tissue T4 during diabetes mellitus is due to the decrease in plasma T4 caused by the decreased thyroidal secretion, possibly due to intracellular energy shortage. The changes in tissue T3 during diabetes mellitus are only partly attributable to the same phenomenon; in several tissues, the decrease in T3 seems more related to the lack of insulin.