Possible Explanations for Patients with Discordant Findings of Serum Thyroglobulin and 131I Whole-Body Scanning

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TABLE 1

Summary of Appropriate Patient Preparation for ¹³¹I DWBS

Appropriate patient preparation
Withdrawal of T₄ for 4–6 wk or of T₃ for 2 wk.
A strict low-iodine diet (≤50 μg iodine per day) followed for 7–14 d before ¹³¹I DWBS and continuing throughout period of imaging.
Avoidance of iodine-containing medications (e.g., iodinated contrast medium, amiodarone, betadine), iodine-rich foods (e.g., kelp), and possible additives of iodine in vitamin and electrolyte supplements.
TSH ≥ 30 mIU/L.
A mild laxative sometimes administered on the evening before ¹³¹I DWBS to simplify image interpretation.
Information relating to patient’s compliance with low-iodine diet, TSH level, history of thyroid hormone withdrawal, measurement of Tg, history of prior administration of contrast medium or iodine-containing drugs (e.g., amiodarone), menstrual history/pregnancy test, nursing/lactation history, etc.
Measurement of urinary iodine in doubtful cases to rule out iodine contamination; repeated WBS 4–6 wk after iodine-depletion regimen such as diuretic program.
Rule out women with pregnancy and breast feeding.

TABLE 2

Summary of Reasons for ¹³¹I DWBS⁻ Tg⁺

Reasons for ¹³¹I DWBS⁻ Tg⁺
False-positive Tg and true-negative ¹³¹I DWBS
Interference of circulating Tg Ab.
Benign lesions (possibly containing thyroiditis) of persistent residual thyroid tissue or nonthyroidal tissue producing Tg.
True-positive Tg and false-negative ¹³¹I DWBS
Defective iodine-trapping mechanism such as acquired inactivation mutation of NIS, TPO gene, and pendrin.
Dedifferentiation of tumor such that it can still produce Tg but has lost its iodine-trapping ability.
Dispersed microscopic metastases too small to be visualized.
Improper patient preparation before ¹³¹I DWBS such as stable iodine contamination and inadequate TSH elevation.