A Study of AL2846 Capsule Versus Placebo in the Treatment of Advanced Radioiodine-Refractory Differentiated Thyroid Carcinoma

Inclusion Criteria: * Participants voluntarily join the study, sign the informed consent form, and demonstrate good compliance. * Histologically or cytologically confirmed locally advanced or metastatic differentiated thyroid carcinoma (DTC). * Age: 18 years ≤ age \<75 years (calculated based on the date of signing the informed consent form). * Eastern Cooperative Oncology Group (ECOG) performance status score: 0-1. * Anticipated survival \>12 weeks. * At least one measurable lesion confirmed by RECIST 1.1 criteria. * Disease progression (per RECIST 1.1) after receiving no more than 2 lines (no more than 3 types) lines of Vascular endothelial growth factor receptor tyrosine kinase inhibitor (VEGFR TKI) therapy * Confirmed iodine-refractory status, defined by \*\*one or more\*\* of the following: 1. Lesions show no iodine uptake on post-Iodine-131 therapy whole-body scan and are unlikely to benefit from further Iodine-131 therapy. 2. Previously iodine-avid lesions progressively lose iodine uptake after Iodine-131 therapy. 3. Mixed iodine-avid and non-iodine-avid lesions in the same patient with no biochemical response. 4. Iodine-avid lesions with disease progression (radiologically confirmed) within 12 months. 5. Cumulative Iodine-131 dose ≥600 mCi (22 GBq) with no disease response (radiologically confirmed). * Thyroid stimulating hormone (TSH) ≤0.5 mIU/L under TSH-suppressive therapy. * Laboratory parameters meeting the following criteria: 1. Hemoglobin (HGB) ≥90 g/L. 2. Absolute neutrophil count (NEUT) ≥1.5×10⁹/L. 3. Platelet count (PLT) ≥90×10⁹/L. 4. Total bilirubin (TBIL) ≤1.5×ULN. 5. Alanine aminotransferase (ALT) and Aspartate transferase (AST) ≤2.5×ULN. 6. Creatinine clearance (CCR) ≥50 mL/min. 7. Prothrombin time (PT), Activated partial thromboplastin time (APTT), and International Normalized Ratio (INR) ≤1.5×ULN (without anticoagulation therapy). 8. Serum albumin (ALB) ≥30 g/L (no albumin infusion within 7 days prior to screening). * For participants of childbearing potential: Agreement to use effective contraception during the study and for 6 months after study completion. Females must have a negative serum/urine pregnancy test within 7 days before enrollment; males must agree to effective contraception during and for 6 months post-study. Exclusion Criteria: * Patients with undifferentiated thyroid carcinoma or medullary thyroid carcinoma; * Patients who have had or currently have other malignancies. The following two situations are eligible for enrollment: other malignancies treated with a single surgery and achieving a disease - free survival (DFS) of 5 consecutive years; cured cervical carcinoma in situ, non - melanoma skin cancer, and superficial bladder tumors \[Ta (non - invasive tumor), Tis (carcinoma in situ), and T1 (tumor invading the basement membrane)\]. * Those with multiple factors affecting oral medications (such as difficulty in swallowing, chronic diarrhea, and intestinal obstruction, etc.); * Adverse reactions from previous treatments have not recovered to a Common Terminology Criteria for Adverse Events (CTCAE) Version 5.0. grade score ≤ 1, except for grade 2 alopecia, grade 2 peripheral neuropathy, grade 2 anemia, non - clinically significant and asymptomatic grade 2 laboratory abnormalities, and hypothyroidism stabilized by hormone replacement therapy, and other toxicities judged by the investigator to have no safety risks. * Known allergy to the excipient components of the study drug. * Subjects who have participated in and used other anti - tumor clinical trial drugs within 4 weeks before randomization. * As judged by the investigator, there are situations that seriously endanger the safety of the subject or affect the subject's completion of the study.