title: Serum Basal thyroglobulin measured by a second-generation assay correlates with the recombinant human thyrotropin-stimulated thyroglobulin response in patients treated for differentiated thyroid cancer.
abstract: BACKGROUND
  Recombinant human thyrotropin (rhTSH) stimulation is frequently used to assess the disease status of patients treated for differentiated thyroid cancer (DTC) when basal (unstimulated) thyroglobulin (b-Tg) is below the assay sensitivity limit. The objective of this study was to determine relationships between the b-Tg and the 72-hour rhTSH-stimulated Tg (rhTSH-Tg) using a second-generation immunochemiluminometric assay with a functional sensitivity of 0.05 ng/mL (microg/L).
  
  
  METHODS
  Serum Tg was measured in paired b-Tg and rhTSH-Tg specimens from 1029 rhTSH tests performed on 849 TgAb-negative patients during long-term monitoring for DTC.
  
  
  RESULTS
  Basal Tg correlated with rhTSH-Tg across b-Tg concentrations ranging from 0.05 to 1000 ng/mL (microg/L) (r = 0.85, p < 0.0001). The b-Tg concentration was unrelated to age, sex, basal TSH, 72-hour TSH, or the Tg fold response (rhTSH-Tg/b-Tg). Further, only 2/655 (0.3%) tests with b-Tg below 0.1 ng/mL (microg/L) had rhTSH-Tg above 2.0 ng/mL (microg/L) (2.9 and 3.8 ng/mL [microg/L], respectively). Thirty-three patients with three or more rhTSH tests performed over a 2- to 5-year period displayed high indexes of individuality for both the 72-hour TSH and the Tg fold response (indexes of individuality = 0.30 and 0.38, respectively). Basal Tg measured using a first-generation assay with a functional sensitivity of 0.9 ng/mL (microg/L) failed to reliably detect an rhTSH-Tg response above 2.0 ng/mL (microg/L).
  
  
  CONCLUSIONS
  An rhTSH-Tg response above 2.0 ng/mL (microg/L) was highly unlikely when b-Tg was below 0.1 ng/mL (microg/L). Second-generation b-Tg measurements correlated with the degree of rhTSH-Tg stimulation and thus the likelihood of having rhTSH-Tg above the customary cut-off of 2.0 ng/mL (microg/L), whereas b-Tg measured by a first-generation assay did not. Correlations between four different assays showed that the use of a fixed Tg cut-off was influenced by assay selection. Patients receiving repetitive rhTSH tests had highly reproducible rhTSH-Tg/b-Tg fold responses, suggesting that repetitive testing is unnecessary and that second-generation measurement of b-Tg trends without rhTSH stimulation would be satisfactory for the long-term monitoring of most patients with DTC.
authors.affiliation: Department of Medicine, Keck School of Medicine, University of Southern California, Los Angeles, California, USA.
authors.forename: Carole
authors.initials: C
authors.lastname: Spencer
authors.forename: Shireen
authors.initials: S
authors.lastname: Fatemi
authors.forename: Peter
authors.initials: P
authors.lastname: Singer
authors.forename: John
authors.initials: J
authors.lastname: Nicoloff
authors.forename: Jonathan
authors.initials: J
authors.lastname: Lopresti
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002454:Cell Differentiation; D006801:Humans; D008163:Luminescent Measurements / Q000379:methods; D008875:Middle Aged; D009364:Neoplasm Recurrence, Local / Q000175:diagnosis; D011863:Radioimmunoassay / Q000379:methods; D011994:Recombinant Proteins; D015203:Reproducibility of Results; D013954:Thyroglobulin / Q000097:blood*; D013964:Thyroid Neoplasms / Q000175:diagnosis* / Q000601:surgery; D013972:Thyrotropin
chemical_list: D011994:Recombinant Proteins; D013972:Thyrotropin; D013954:Thyroglobulin