Parasitic thyroid nodules: cancer or not?

In 2006, a 58-year-old woman presented with thyrotoxicosis. She had undergone left hemithyroidectomy 14 years before for a benign follicular adenoma. Ultrasound imaging demonstrated bilateral cervical lymphadenopathy with enhanced tracer uptake in the left lateral neck on a Technetium-99m uptake scan. Fine-needle aspiration biopsy of a left lateral neck node was insufficient for a cytological diagnosis; however, thyroglobulin (Tg) washings were strongly positive. The clinical suspicion was of functionally active metastatic thyroid cancer in cervical lymph nodes. A completion thyroidectomy and bilateral cervical lymph node dissection were performed. Histology demonstrated benign multinodularity in the right hemithyroid, with bilateral reactive lymphadenopathy and 24 benign hyperplastic thyroid nodules in the left lateral neck that were classified as parasitic thyroid nodules. As there had been a clinical suspicion of thyroid cancer, and the hyperplastic/parasitic thyroid tissue in the neck was extensive, the patient was given ablative radioactive iodine (3.7 GBq). After 2 years, a diagnostic radioactive iodine scan was clear and the serum Tg was undetectable. The patient has now been followed for 7 years with no evidence of recurrence. Archived tissue from a left lateral neck thyroid nodule has recently been analysed for BRAF V600E mutation, which was negative. LEARNING POINTS: Thyrotoxicosis due to functional thyroid tissue in the lateral neck is very rare and may be due to metastatic thyroid cancer or benign parasitic thyroid tissue. Parasitic thyroid nodules should be considered as a differential diagnosis of lateral neck thyroid deposits, particularly where there is a history of prior thyroid surgery. Parasitic thyroid nodules may occur as a result of traumatic rupture or implantation from a follicular adenoma at the time of surgery. The use of ablative radioactive iodine may be appropriate, as resection of all parasitic thyroid tissue can prove difficult. BRAF mutational analysis of parasitic thyroid tissue may provide extra reassurance in the exclusion of papillary thyroid carcinoma.