The availability of sensitive thyrotropin assays allows effective biochemical monitoring of both replacement and suppressive therapy with thyroxine. Whatever target organ is examined, there is tissue thyrotoxicosis if the serum thyrotropin concentration is low, even if the serum triiodothyronine and thyroxine concentrations are normal. Although suppression of thyrotropin secretion is recommended in the treatment of patients with thyroid carcinoma, the aim of thyroxine-replacement therapy in patients with primary hypothyroidism should be to maintain the serum thyrotropin concentration in the normal range. The most convincing argument for the treatment of subclinical hypothyroidism is progression to overt hypothyroidism at a rate of 5 to 20 percent per year.