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Incidence and implications of negative serum thyroglobulin but positive I‐131 whole‐body scans in patients with well‐differentiated thyroid cancer prepared with rhTSH or thyroid hormone withdrawal

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Aims  To evaluate the incidence and clinical implications of a positive whole‐body I‐131 scan but negative stimulated serum Tg/TgAb level following an ablative or diagnostic I‐131 dose in patients with well‐differentiated thyroid cancer and whether there is a difference in incidence if prepared with thyroid hormone withdrawal compared with rhTSH stimulation.

Methods  I‐131 scan findings, serum Tg/TgAb levels, TNM stage and method of thyroid tissue stimulation in 193 consecutive patients (138F, 55M) with well‐differentiated thyroid cancer undergoing postoperative ablative I‐131 therapy and 121 consecutive (94F, 27M) patients undergoing diagnostic I‐131 surveillance scans were retrospectively reviewed. Comparisons of proportions were performed using Chi‐square tests. Clinical, biochemical and I‐131 scan follow‐up data were obtained for each patient cohort.

Results  39/193 (20·2%) postablative I‐131 and 10/121 (8·3%) diagnostic I‐131 patients had negative stimulated serum Tg/TgAb levels but positive I‐131 scans for residual thyroid tissue. Nine (4·7%) of the postablative patients had I‐131 uptake in the lateral neck suspicious for loco‐regional metastatic disease. In the postablative I‐131 group, 38/169 (22·5%) prepared with rhTSH compared to 1/24 (4·2%) prepared with thyroid hormone withdrawal were Tg/TgAb negative but I‐131 scan positive (P = 0·04). Follow‐up of 21/39 postablative I‐131 patients with negative Tg/TgAb but positive I‐131 scans confirmed a significant proportion of patients (4/21) (19·1%), remained Tg/TgAb negative/I‐131 scan positive, some of whom had higher‐risk disease at original diagnosis (2/4) (50%).

Conclusions  Our study confirms that in the setting of I‐131 ablation therapy or diagnostic I‐131 scanning, a significant proportion of patients (20·2% and 8·3%, respectively) have residual benign or malignant thyroid tissue on whole‐body scanning despite a negative stimulated serum Tg level. Whether such patients who would otherwise be missed as having residual thyroid tissue on serum Tg testing alone have a worse clinical outcome remains uncertain. Our findings do however suggest performing both stimulated serum Tg/TgAb levels and I‐131 scans for the follow‐up of patients with higher‐risk thyroid cancer may be important. There may also be a slightly higher incidence of this phenomenon in patients prepared with rhTSH rather than by thyroxine withdrawal.
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Document Type: Research Article

Affiliations: 1: Department of Nuclear Medicine 2: Monash University Department of Epidemiology & Preventive Medicine, Alfred Hospital, Melbourne, Vic., Australia

Publication date: May 1, 2012

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