Endocrine Abstracts

Introduction

Thyrotoxicosis due to functioning metastases in differentiated thyroid cancer (DTC) is exceedingly rare. The mechanism remains unknown despite several hypotheses. We report on a case of transient thyrotoxicosis due to functioning bone metastasis of follicular thyroid carcinoma.

Case report

We present a case of female patient of 68-years-old diagnosed with follicular thyroid carcinoma revealed by a lytic lesion of the right iliac bone. She had undergone total thyroidectomy followed by treatment with 100 mCi of iodine 131.The post-therapeutic whole-body scan showed intense uptake in the right iliac bone and residual thyroid gland uptake. Bone scintigraphy and SPECT/CT fusion images showed focal, intense uptake in the right iliac bone. Thoracic computed tomography and skeletal scintigraphy revealed bilateral multiple nodules in her lungs. Initially, TSH levels after 4 weeks of stopping levothyroxine treatment in preparation for treatment with I-131were high (> 60 mU/l; normal range, 0.25–5 mU/ml). The patient had undergone partial surgical resection of approximately 50% of the volume of the right iliac tumor mass.The bone metastasis was much too extensive and hypervascular to permit complete resection. After surgical management, the patient complained of symptoms of hyperthyroidism. The hyperthyroidism was confirmed by laboratory tests. Thyroid stimulating hormone (TSH) remained suppressed and free T4 remained elevated after I-131 therapy without thyroid hormone supplementation. The levothyroxine remained discontinued. TSH returned to adequate levels 8 months after surgery. TSH levels increased to 128 mU/l. Levothyroxine treatment was restarted at a suppressive dose of 175 µg/ml.

Conclusion

Thyrotoxicosis caused by thyroid follicular metastases is principally the consequence of multiple metastatic localizations, with a large tumoral volume. It must be evoked when there is no increase of the TSH after a thyroidectomy, or during the period of interruption of the suppressive treatment.