We present a fascinating and instructive case of unexplained suppression of thyroglobulin (Tg) levels following resection of a presumed carcinoid tumour, subsequent re-examination of which revealed it to be a solitary metastasis from a follicular thyroid carcinoma.
The patient, RB is a 79-year-old woman who presented 11 years ago with a 2-week history of neck swelling. Examination revealed a 5 cm diameter nodule in the left lobe of the thyroid, confirmed on ultrasound scan (with no specific diagnostic features). Fine needle aspiration cytology was suspicious for but not diagnostic of follicular thyroid carcinoma. She had a left total thyroidectomy. Following histological confirmation of a minimally-invasive follicular thyroid carcinoma, completion thyroidectomy (negative for tumour) and subsequent radio-iodine ablation therapy were performed.
Her post-operative Tg levels were initially undetectable and she remained asymptomatic and euthyroid on thyroxine replacement. After 7-years post-thyroidectomy, her Tg levels started to rise gradually over the course of a year up to a level of 60 mg/l. MRI scan showed no evidence of tumour recurrence within the thyroid bed. Although whole-body I125 scan showed no uptake, a CT scan showed a solitary 17mm-diameter pulmonary nodule at the right lower lobe (also present on an octreotide scan). A CT-guided biopsy of her pulmonary nodule 2-years ago was followed by thoracoscopic complete excision. The lesion showed features typical for a carcinoid tumour on both histology and immunohistochemistry. Following resection of a presumed incidental pulmonary carcinoid lesion, her Tg levels dropped rapidly to undetectable levels and have remained undetectable. Recent re-examination of the pulmonary nodule revealed strong positivity for Tg and features consistent with a metastatic follicular thyroid carcinoma.
This case illustrates the potential for mis-diagnosis on the basis of histological examination, and the occasional need for histological re-examination in the context of inconsistent biochemistry.