title: Does a highly sensitive thyroglobulin (Tg) assay change the clinical management of low-risk patients with thyroid cancer with Tg on T4 < 1 ng/ml determined by traditional assays?
abstract: OBJECTIVE
  To evaluate a highly sensitive thyroglobulin (Tg) assay [functional sensitivity (FS): 0.1 ng/ml] (Tg-ICMA) in low-risk patients with known Tg on T4 < or = 1 ng/ml measured by a traditional assay (FS: 1 ng/ml) (Tg-IRMA).
  
  
  METHODS
  Tg-ICMA was measured in serum samples stored at -70 degrees C. Samples were obtained 6 months or more after total thyroidectomy and remnant ablation with (131)I, during L-T4 therapy (TSH < 0.4 mIU/l). All patients had well-differentiated and completely resected tumours, no ectopic uptake on post-therapy whole-body scans and were considered to be at low risk for recurrence. On the occasion of collection and retesting for this study, Tg-IRMA was < or = 1 ng/ml in all samples and no antibody interference was observed.
  
  
  RESULTS
  Tg-ICMA < or = 0.1 ng/ml was observed in 130/178 (73%) patients and recurrence was diagnosed in only 1/130 (0.8%). Tg-IRMA measured after L-T4 withdrawal was > 1 ng/ml in 5/130 (3.8%) patients. Forty-eight (27%) patients had Tg-ICMA > 0.1 ng/ml (0.12-1.6 ng/ml) and recurrence was diagnosed in 5/48 (10.5%). Tg-IRMA measured after L-T4 withdrawal was > 1 ng/ml in 20/48 (41.6%) patients. A negative predictive value of 100% was achieved with Tg-ICMA on T4 < or = 0.1 ng/ml combined with neck ultrasonography (US) or with stimulated Tg-IRMA < or = 1 ng/ml.
  
  
  CONCLUSIONS
  Patients at low risk for recurrence with undetectable Tg on T4 measured by a highly sensitive assay (FS: 0.1 ng/ml) in the absence of antibody interference and with a negative sensitive neck US do not need to be submitted to Tg stimulation. Recurrence is rare in these cases and only a minority of patients convert to stimulated Tg > 1-2 ng/ml.
authors.affiliation: Department of Thyroid, Endocrinology Service, Santa Casa de Belo Horizonte, Minas Gerais, Brazil. pedrorosario@globo.com
authors.forename: P W
authors.initials: PW
authors.lastname: Rosario
authors.forename: S
authors.initials: S
authors.lastname: Purisch
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D019411:Clinical Laboratory Techniques / Q000379:methods; D019468:Disease Management; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009364:Neoplasm Recurrence, Local / Q000175:diagnosis*; D012307:Risk Factors; D012680:Sensitivity and Specificity; D013954:Thyroglobulin / Q000097:blood*; D013964:Thyroid Neoplasms / Q000175:diagnosis / Q000628:therapy*; D013974:Thyroxine / Q000097:blood*; D055815:Young Adult
chemical_list: D013954:Thyroglobulin; D013974:Thyroxine