title: Adequate thyroid-stimulating hormone levels after levothyroxine discontinuation in the follow-up of patients with well-differentiated thyroid carcinoma.
abstract: BACKGROUND
  In the follow-up of patients with well-differentiated thyroid carcinomas (WTC), a thyroid-stimulating hormone (TSH) >or=30 micro U/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 I) and after thyroidectomy in 10 subjects (group II). WBS and Tg determination was performed upon achievement of TSH >or=30 micro U/mL.
  
  
  RESULTS
  By the second week, 42% of group I patients and 70% of group II patients had TSH >or=30 micro U/mL. By the third week, 90% in group I and 100% 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.
authors.affiliation: Servicio de Endocrinología, Unidad de Investigación Médica en Endocrinología Experimental, Hospital de Especialidades, Centro Médico Nacional Siglo XXI (CMN-SXXI), Instituto Mexicano del Seguro Social (IMSS), Mexico City, Mexico.
authors.forename: Reyna
authors.initials: R
authors.lastname: Sánchez
authors.forename: Ana Laura
authors.initials: AL
authors.lastname: Espinosa-de-los-Monteros
authors.forename: Victoria
authors.initials: V
authors.lastname: Mendoza
authors.forename: Eduardo
authors.initials: E
authors.lastname: Brea
authors.forename: Irma
authors.initials: I
authors.lastname: Hernández
authors.forename: Ernesto
authors.initials: E
authors.lastname: Sosa
authors.forename: Moisés
authors.initials: M
authors.lastname: Mercado
mesh_terms: D000328:Adult; D002454:Cell Differentiation; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D008875:Middle Aged; D011877:Radionuclide Imaging; D013964:Thyroid Neoplasms / Q000000981:diagnostic imaging* / Q000188:drug therapy* / Q000473:pathology; D013972:Thyrotropin / Q000097:blood*; D013974:Thyroxine / Q000097:blood / Q000494:pharmacology*; D013997:Time Factors; D014284:Triiodothyronine / Q000097:blood
chemical_list: D014284:Triiodothyronine; D013972:Thyrotropin; D013974:Thyroxine