A 36-year-old woman who underwent total thyroidectomy and radioiodine ablation for papillary carcinoma is on suppressive levothyroxine. Her stimulated serum thyroglobulin remains undetectable, but antithyroglobulin antibodies are strongly positive. The correct interpretation of her thyroglobulin result is that it may be:

• A Falsely high, because antibody complexes are counted as thyroglobulin
• B Falsely low, because circulating autoantibodies interfere with the assay ✓
• C Accurate, since modern immunometric assays are unaffected by autoantibodies
• D Uninterpretable only if the patient is hypothyroid at the time of sampling

Explanation

After complete ablation, thyroglobulin should be produced only by residual or recurrent thyroid tissue, making it an ideal post-surgical marker. However, antithyroglobulin antibodies interfere with immunometric assays and cause falsely low readings, potentially masking recurrence, so antibody status must be measured alongside every thyroglobulin value. Option C is wrong because this interference persists even with current assays, and TSH status affects true secretion levels rather than assay interpretability alone.

Reference: Williams Textbook of Endocrinology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.