USC ENDOCRINE LABORATORIES

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TEST INDEX


Thyroglobulin (Tg) Tg Antibody TSH Free T4 Total T4 Free T3 Total T3 TPO Antibody
THBR FT4I or FT3I TBG Calcitonin PTH


THYROGLOBULIN (Tg)

Methodology  Immunochemiluminometric assay (ICMA) for Antibody Negative Sera/FNAW
                        Radioimmunoassay (RIA) for Antibody Positive Sera/FNAW

Performed  Mon - Fri (Antibody Negative Sera/FNAW)
                    Tues - Fri (Antibody Positive Sera/FNAW)

Reported  Next Day (ICMA) / 6 days after setup (RIA)

Collect  One 6mL RT (Min: 4 mL RT)

Transport  2 mL refrigerated serum or FNAW (Min: 1.5 mL)

Stability  Stored at 2 - 8°C until used; Stable if frozen at - 20°C.  Studies indicate that Tg in serum is remarkably stable even at ambient temperature if there are no losses due to evaporation.  Samples are usually stored at 2 - 8°C for up to one week. For long-term storage, freezing samples at -10 to -20°C is preferred. Repeat thawing and freezing of samples does not influence values if there are no losses due to evaporation.

Unacceptable  The best qualitative result is obtained if the use of grossly hemolyzed or lipemic samples is avoided.

Remarks:  The NACB Guidelines state that any laboratory testing for Thyroglobulin MUST also determine the TgAb status of that patient’s sera.  The new NACB guidelines also suggest that laboratories archive specimen left after serum Tg measurement for at least 6months.  This allows for concurrent re-measurement of the past and current specimens in the same run - a maneuver that eliminates the between-run error and improved the clinical sensitivity of the test.  Our lab stores Tg sera indefinitely. *(Thyroid 13:57-67, 2003)

Reference Range for FNAW  <1.0 ng/mL

Reference Range for Serum      
*TG REFERENCE RANGES WITH TSH CORRELATION TG TSH
Normal Thyroid (3 – 40 ng/mL) (0.3 - 3.0 mIU/L)
Normal Thyroid (suppressed TSH) (1.5 – 20 ng/mL) (<0.1 mIU/L)
Post Lobectomy (suppressed TSH) (<10 ng/mL) (<0.1 mIU/L)
Near-Total Thyroidectomy (suppressed TSH) (<2 ng/mL) (<0.1 mIU/L)
Athyreotic (TSH Stimulated) (<1 ng/mL) (>20 mIU/L)
Goiter (Thyroid enlargement) (>40 ng/mL) -

CPT Code(s)  84432 (Tg)        84432-59 (TGP = concurrent Tg re-measurement of a previous sample)

Interpretive Data: Thyroglobulin (Tg) is routinely measured with a chemiluminometric assay (ICMA).  Because immunometric assay methodology underestimates serum Tg when TgAb is present, samples with detectable TgAb results are reflexed to the RIA method. (See USC Thyroglobulin Testing Strategy)

The RIA method measures both free and antibody-bound Tg and is minimally affected by TgAb.  However, TgAb may interfere and result in falsely low or high Tg levels.  TgAb levels may increase after radioiodine treatment and decrease after successful surgery.  Total Tg is a useful tumor marker if TgAb levels remain constant. 

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