Oxidative stress is involved in the pathogenesis of thyroid cancer, but the mechanism is not clear. The thyroid is the organ with the most abundant selenium content, and selenium may be involved in protecting the gland from the influence of large amounts of H2O2 produced during thyroid hormone biosynthesis.

Selenium may exert anti-tumor activity through a variety of mechanisms, including inducing apoptosis and anti-oxidation to change the DNA methylation state of tumor suppressor genes, cell cycle arrest and stimulation of the immune system, as well as playing an anti-tumor role through its anti-inflammatory and anti-angiogenesis properties.

The whole blood and thyroid selenium concentrations in patients with thyroid cancer were lower, and the decreased serum selenium levels were also associated with the high TNM stage of thyroid cancer.

According to the Nutrition Prevention of Cancer (NPC) trial, selenium yeast supplements with a daily selenium content of 200 MCG have been shown to reduce the incidence of total cancer, prostate cancer, colon cancer, and lung cancer, and cancer mortality. The active agent in selenium yeast supplements is known as selenium methionine (SEMET).

In general, the association between selenium and thyroid cancer is still inconclusive, the question of whether low selenium is a predisposition factor or a consequence of thyroid cancer has not been resolved, and the clinical effect of selenium supplementation in preventing thyroid cancer or improving its prognosis remains to be studied.

The hypothesis is that supplementation with selenium yeast will improve the prognosis of patients with differentiated alpha-carcinoma.

Inclusion Criteria:

1. Participants have signed informed consent forms；
2. Patients with differentiated thyroid carcinoma diagnosed pathologically after thyroid surgery；
3. Male or female patients aged 18-75 years;
4. Women who are likely to become pregnant must use the appropriate contraceptive method to avoid pregnancy and minimize the likelihood of conception between the beginning of the drug intervention study and the 28th day after the study.

Exclusion Criteria:

1. A patient who is pregnant or breastfeeding;
2. Currently, hepatase cytochrome P450 3A4 induction or inhibitor therapy, antiviral therapy for immunodeficiency diseases (note: hepatase induction or inhibitor: phenobarbital phenobarbital sodium rifampicin carbamazepine grisoflomycin and dexamethasone and chloramphenicol allopurinol ketone conazole isoniazid imittidine phenothiazine);
3. Gastrointestinal surgery that may affect the study of drug absorption;
4. The patient has a history of haemoglobin disease or acute progressive nephropathy or autoimmune skin disease;
5. A history of substance abuse and alcohol abuse within the last 1 year;
6. There are therapeutic contraindications with selenium yeast capsules as listed in the instructions;
7. New York Heart Association (NYHA) class III or IV congestive heart failure and/or left ventricular ejection fraction of 40% with a significant cardiovascular history in the past 6 months: myocardial infarction coronary angioplasty or bypass surgery valvular disease or repair of unstable angina transient ischemic attack or cerebrovascular accident;
8. There are obvious abnormalities in liver function;
9. The patient has significant liver disease acute active hepatitis or chronic active hepatitis clinical signs or symptoms；
10. Laboratory and physical examination or ECG findings of any clinically significant abnormality would, in the investigator's judgment, compromise the patient's safety or prevent successful participation in the clinical study；
11. Patients with severe renal insufficiency.

All participants, investigators were blinded to the grouping of the experiment and the intervention.