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Purposes: Distant metastasis from papillary thyroid microcarcinoma (PTMC) is extremely rare and the long-term outcomes and independent prognostic factors remain unclear. The present study aimed to investigate clinicopathological characteristics and evaluate the long-term outcomes and prognostic factors of PTMC patients with distant metastases (DM) who underwent surgery and radioactive iodine (131I) treatment. Methods: We retrospectively reviewed the medical records of 13,441 patients with thyroid cancer (including 1697 cases with PTMC) who underwent 131I treatment at our institution between January 2008 and December 2019. PTMC patients with distant metastases with sufficient clinical follow-up data were enrolled in this cohort study. The overall survival (OS) and progression-free survival (PFS) were analyzed by the Kaplan–Meier method and the prognostic factors were assessed by Cox proportional hazards. Results: Thirty-three PTMC patients with DM were enrolled in this study. The median follow-up was 75 months (range: 5–151 months). The 5-year and 10-year OS rates were 96.97 and 81.41%, respectively, and the 5-year and 10-year PFS rates were 90.46 and 69.68%, respectively. Multivariate analysis showed that male sex (P= 0.005), radioactive iodine refractory PTMC (P= 0.033), and symptomatic DM (P= 0.022) were significantly associated with worse 10-year PFS in PTMC patients with DM. No independent predictor related to poor 10-year OS was found in the present study. Conclusions: The prognosis of PTMC patients becomes worse after the development of DM. Male sex, radioactive iodine refractory PTMC, and symptomatic DM were identified as independent factors associated with PFS.

Metastatic differentiated thyroid carcinoma (DTC) with positive 131I scintigraphy, but negative stimulated Tg (sTg) is relatively rare in clinical practice. The clinical characteristics of these patients were analyzed in the current study. A total of 3367 consecutive histologically proven DTC patients were analyzed retrospectively from January 2007 to June 2013. Tg negativity was defined as a sTg level of <2 ng/mL without positive anti-Tg antibody (TgAb level of <100 IU/mL) under thyroid-stimulating hormone stimulation (TSH level of ≥30 mIU/L). Analyses were performed using the Statistical Package for the Social Sciences, version 20.0 (SPSS, Chicago, IL, USA). Seventy-one patients (median age 45 years, range 17–68 years) were post-therapeutic 131I-SPECT/CT positive and sTg negative (PTP-TN) constituting 2.1 % of all patients. Of these 71 patients, 2 (2.8 %) had bone metastasis, 11 (15.5 %) had lung metastasis, and 59 (83.1 %) had lymph node metastasis. Fifty-six patients had cervical lymph node metastasis (cLNM), and US was positive in 15 patients (26.8 %), while negative in 41 patients (73.2 %). When compared to patients with concordant positive results for sTg and 131I scintigraphy, US showed a relatively lower positive rate in the detection of cLNM in PTP-TN patients (28.8 vs. 53.8 %; χ2= 6.70; P= 0.01). In conclusion, even with sTg <2 ng/mL, there is a low risk of metastatic DTC. US had limitations in PTP-TN patients, while post-therapy 131I-SPECT/CT demonstrated an advantage in the detection of functioning metastasis despite low sTg levels in patients with metastatic DTC.