In acute and subacute disease the active thyroid hormones, triiodothyronine (T3) and tetraiodothyroacetic acid (tetrac), are decreased while serum-thyroxine (T4) levels tend to be slightly reduced. Conversely, the inactive metabolite, reverse triiodothyronine (reverse T3), is increased indicating a diversion of T4 metabolism from an activating to an inactivating pathway. With convalescence the serum levels of T3, tetrac, and T4 recover while reverse T3 decreases to normal. These changes occur without significant alterations in serum levels of thyroid-stimulating hormone, indicating maintenance of euthyroidism throughout disease.