Adequate thyroid-stimulating hormone levels after levothyroxine discontinuation in the follow-up of patients with well-differentiated thyroid carcinoma

Background. In the follow-up of patients with well-differentiated thyroid carcinomas (WTC), a thyroid-stimulating hormone (TSH) greater than or equal to30 muU/mL is generally accepted as adequate to perform whole body scans (WBS), determine thyroglobulin (Tg), and administer radioiodine therapeutically. These patients, inevitably rendered hypothyroid, are traditionally switched to T3 for 3-4 weeks prior to withdrawing all thyroid hormones for an additional 2-3 weeks. Neither TSH and Tg elevation dynamics nor WBS characteristics after simply interrupting L-T4 treatment without T3 administration have been evaluated. Methods. TSH, total T4 and T3, as well as FT4 were measured weekly after discontinuing L-T4 in 21 subjects (group 1) and after thyroidectomy in 10 subjects (group II). WBS and Tg determination was performed upon achievement of TSH greater than or equal to30 muU/mL. Results. By the second week, 42% of group I patients and 70% of group II patients had TSH greater than or equal to30 muU/mL. By the third week, 90% in group I and 1,00% in group II had achieved this target. Group I patients who needed 4 weeks to increase TSH received a greater cumulative radioiodine dose and had higher Tg levels. Positive WBS were found in eight cases and the incidence of a negative WBS with elevated Tg was significantly higher when evaluation occurred at the second week of L-T4 withdrawal compared to the fourth week. Conclusions. L-T4 interruption is a reasonable alternative to temporary T3 in preparation for radioiodine scanning and treatment. (C) 2002 IMSS. Published by Elsevier Science Inc.