A significant role in improving the prognosis of differentiated thyroid cancer (DTC) in the presence of bone metastases is determined by early diagnosis of metastases, timely and correctly selected treatment tactics for the patient. During dynamic follow-up of patients with DTC after combined treatment (thyroidectomy with radioiodine therapy) are determination of the level of oncomarkers (serum thyroglobulin and antibodies to thyroglobulin) and ultrasound diagnostic of the neck, scanning with radioactive iodine (if clinically indicated). In some cases, patients have TENIS-syndrome (Thyroglobulin Elevated Negative Iodine Scintigraphy, hereinafter TENIS-syndrome), characterized by high serum thyroglobulin level in blood and absence of radioactive iodine accumulation on post therapeutic scintigraphy. According to the research studies, PET/CT with 18F-FDG has high sensitivity and specificity (89 % and 72 %, respectively) in visualization of metastatic radioiodine refractory foci in TENIS-syndrome.

This article presents a clinical case of a patient, a 52-year-old woman with Gurtle cell thyroid cancer (pT3aN0M0, stage I) with established TENIS syndrome. Thyroidectomy was performed in September 2019 and radioiodine therapy was performed in January 2022 due to suspected disease progression given high thyroglobulin levels. Given the absence of pathologic accumulation of 131I according to post-therapy radioiodine scanning, PET/CT with 18F-FDG was performed, which revealed a solitary metastasis in the left iliac bone (41×35×42 mm with SUVmax 17.25). In November 2022, radical treatment of the solitary bone metastasis was performed in the scope of resection of the left iliac bone with reconstructive-plastic component. According to the data of control examinations in June 2023, the patient has a complete biochemical and radiologic remission of the disease.