Levothyroxine therapy and thyrotropin suppression

Abstract

Levothyroxine (L-T4) is arguably the most prescribed medication in the United States, with its primary therapeutic indication for hypothyroidism from chronic thyroiditis or Hashimoto's disease. Such patients are candidates for so-called replacement dosage, with the administered dosage of L-T4 titrated to a target thyrotropin (TSH) level within the normal range of 0.4-2.0 mU/L. Patients with thyroid cancer who have had near-total to total thyroidectomy are usually candidates for suppressive levothyroxine dosage, which is so-called because the aim of therapy is to give a slightly supraphysiologic dosage of thyroxine to suppress TSH. The rationale for suppressive therapy is based on studies indicating that TSH stimulation enhances tumor growth, TSH serving as a growth factor or mitogen for thyroid malignancies with observations of more rapid tumor growth seen clinically after thyroxine withdrawal. The growth-promoting property of TSH is presumed to be from the presence of TSH receptors on thyroid cancer cells (1); however, non-TSH receptor-mediated growth is certainly a property of undifferentiated thyroid cancers. © 2006 Humana Press Inc.