Inclusion Criteria:
* Aged 18-65 years, inclusive, regardless of sex.
* Able to fully understand the study and voluntarily sign the informed consent form.
* Histologically or cytologically confirmed advanced or metastatic thyroid cancer, treatment-naïve, and requiring total or near-total thyroidectomy.
* Eastern Cooperative Oncology Group (ECOG) performance status ≤2.
* No clinically significant contraindications to surgery.
* Body weight ≥50.0 kg for males and ≥45.0 kg for females, with a body mass index (BMI) of 19.0-26.0 kg/m², inclusive.
* Able to maintain regular daily routines, communicate effectively with the investigator, comply with all study requirements, and adhere to scheduled study visits, treatment plans, and laboratory assessments.
* Adequate baseline organ function at the screening visit, as demonstrated by local laboratory values meeting the protocol-specified eligibility requirements.
Exclusion Criteria:
* Thyroid cancer patients for whom the planned or completed thyroidectomy does not meet the criteria for total or near-total thyroidectomy.
* Prior treatment for thyroid cancer, including surgery and/or pharmacological treatment.
* Major surgery, including radiosurgery, or significant traumatic injury within 2 weeks prior to randomization.
* History of another malignancy within 3 years prior to initiation of study treatment.
* Known history of a positive human immunodeficiency virus (HIV) test.
* Known active hepatitis B virus (HBV) or hepatitis C virus (HCV) infection; chronic or resolved HBV or HCV infection is permitted.
* Congenital or acquired immunodeficiency.
* History of clinically significant drug allergy or atopic allergic disease (e.g., asthma, urticaria, or eczematous dermatitis), or known hypersensitivity to Euthyrox (levothyroxine sodium) or any of its excipients.
* Pregnant or breastfeeding females.
