To determine the long term effects of a protein sparing fast on serum thyroid hormone levels, the authors studied 38 obese patients ingesting a diet of 320 kcal for up to 13 weeks. The high baseline serum triiodothyronine (T3) levels decreased significantly by the first week, further decreased by the third week, and this lower level persisted for the duration of the fast until realimentation. Serum free T3index followed the same general pattern as did serum T3levels. Serum reverse T3increased significantly by the first week, but by week three, the reverse T3level had begun to fall, although still significantly increased above baseline. By week seven, reverse T3had decreased to almost baseline and remained not significantly changed from the baseline to 13 weeks. Serum thyroxine (T4) increased significantly by the first week in all patients, but by the third week had returned to baseline levels which persisted to 13 weeks. The free T4index and free T4concentrations showed the same increment at week one and then returned to baseline levels. There were no significant changes in serum thyroxine-binding globulin (TBG) or thyroid-stimulating hormone (TSH) concentrations. The changes in serum T3and reverse T3levels are attributable to alterations in peripheral 5′-monodeiodination of T4 and reverse T3induced by the protein sparing fast.