MON-399 - A Case of High Post-Operative Thyroglobulin after Thyroidectomy for Low-Risk Papillary Thyroid Carcinoma Explained by Adnexal Radioiodine Uptake

Background:
Papillary thyroid carcinoma (PTC) has a low risk for metastasis, especially when small and localized. The differential for elevated post-operative thyroglobulin (TG) includes residual normal thyroid tissue, ectopic thyroid tissue, and residual or metastatic carcinoma. Workup includes additional imaging and radioactive iodine-131 (RAI) ablation. RAI uptake outside the thyroid bed indicates ectopic thyroid tissue, metastatic carcinoma, or physiologic uptake. We present a case of an unexpectedly elevated TG after surgery for low-risk PTC with isolated adnexal uptake of RAI and resolution of thyroglobulinemia after treatment.

Case Report:
A 40-year-old female with a history of scar tissue endometriosis was diagnosed with an incidental thyroid nodule and underwent fine-needle aspiration biopsy which resulted as “suspicious for PTC.” She had a total thyroidectomy by a high-volume endocrine surgeon which confirmed a low risk-of-recurrence PTC with no adverse histopathologic features. Her post-operative TG was high at 2.10 ng/mL and TSH was 0.32 uIU/mL. Neck ultrasound with cervical node mapping did not reveal any suspicious findings or residual thyroid tissue. Remnant ablative RAI was performed after she completed breastfeeding, 14 months after thyroidectomy. Post-treatment whole body imaging demonstrated intense thyroid bed radiotracer uptake, along with low intensity uptake in the left adnexal region. Pelvic ultrasound revealed a normal left ovary and a solid 3.6cm likely endometrioma in the surrounding adipose. Her serum TG decreased following RAI, suggesting residual and/or ectopic ovarian thyroid tissue as the source.

Discussion:
The differential for an elevated post-op TG with adnexal RAI uptake includes ectopic thyroid tissue, uptake in cystic lesions like cystadenoma, contamination, or spurious interpretation of physiologic distribution; the ovary is not a usual site of metastasis. Ectopic thyroid tissue is detectable in < 5% of teratomas although struma ovarii, ovarian masses harboring teratomas with >50% thyroid tissue, are rarer, and fewer than 20% of struma ovarii are associated with hyperthyroidism. The normal appearance of the ovaries on gynecological ultrasound scan makes struma ovarii less likely. To date, there are no reported cases of iodine uptake in solid endometriomas, only cystic endometriomas. We suspect uptake in an area of ectopic thyroid tissue below the size threshold to be considered a struma ovarii, although we can only speculate to what extent it was contributing to the high post-op TG.

Conclusion:
Residual thyroid tissue should be considered as a potential contributor to elevated TG after thyroidectomy for low-risk PTC and can be treated with remnant ablative RAI; uptake in the ovary on whole body scanning is uncommon and likely reflects ectopic thyroid tissue.

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