Differentiated thyroid cancer: comparison of therapeutic iodine 131 biological elimination after discontinuation of levothyroxine versus administration of recombinant human thyrotropin

The biological elimination of therapeutic 131I in patients with differentiated thyroid cancer (DTC), post total or near-total thyroidectomy, was compared after withholding levothyroxine suppression against administration of recombinant human thyrotropin without stopping levothyroxine. In 163 patients (group G1) levothyroxine was withheld before 131I therapy: in 138 patients the tumor was limited to the thyroid bed (group G1.1) and in 25 patients metastases were present (group G1.2). A second group of patients (G2; n = 28) received 131I therapy after administration of recombinant human thyrotropin without stopping levothyroxine. Mean retained 131I activity (as a percentage of the administered dose) was 5%-29% (group G1.1), 20%-43% (group G1.2) and 1%-17% (group G2). The effective half-life of 131I was 0.59-0.69 days (group G1.1), 0.87-1.22 days (group G1.2) and 0.38-0.44 days (group G2). In conclusion, the use of recombinant human thyrotropin to prepare patients with thyroid cancer for therapy with 131I shortens its effective half-life and reduces its retained activity compared to preparation with discontinuation of levothyroxine suppression.