The global incidence of Differentiated Thyroid Cancer (DTC) is rising. While surgery followed by TSH suppression is the standard of care, achieving target TSH levels with levothyroxine (L-T4) monotherapy remains challenging, with only 25-70% of intermediate/high-risk patients attaining it within 6-8 months. This therapeutic dilemma stems from three key issues: impaired T4-to-T3 conversion due to DIO2 polymorphisms, the non-physiological hormone ratio of T4 monotherapy, and L-T4's narrow therapeutic window. This often results in an "under- versus over-suppression" paradox, increasing risks of recurrence, atrial fibrillation, and osteoporosis. Combining L-T4 with desiccated thyroid extract (DTE; T4:T3 ≈ 4:1) may overcome these limitations by bypassing DIO2 defects and providing a more physiological hormone profile, thereby potentially improving TSH control while mitigating side effects. Supported by the 2023 Chinese guidelines and our promising pilot data (82% cumulative target attainment at a median of 1.4 months), we propose a two-stage national study: a multicenter cohort study followed by a randomized trial, to generate high-level evidence for this combination therapy in high-risk DTC.

This study is a multicenter, open-label, randomized controlled trial. Stage 1 eligibility: (1) Age ≥ 18 years；(2) Histologically proven DTC after initial surgery (± ¹³¹I)；(3) ATA recurrence risk intermediate (low-intermediate or high-intermediate) or high；(4) On L-T4 monotherapy: TSH above target despite FT4 ≥ 90 % of normal upper limit；(5) Signed informed consent and agreement to follow-up. Stage 2 eligibility: (1) Age 18-70 years (inclusive)；(2) Histologically proven DTC undergoing first-time surgery；(3) ATA recurrence risk intermediate (low-intermediate or high-intermediate) or high；(4) Signed informed consent and agreement to post-operative follow-up. Exclusion criteria: (1) Non-DTC histology; (2) Hyperthyroidism, toxic adenoma, or toxic nodular goiter; (3) Malabsorptive GI disorders (e.g., H.-pylori gastritis, atrophic gastritis, celiac disease) or history of gastric bypass; (4) Hypersensitivity to levothyroxine or desiccated thyroid; (5) Angina, coronary artery disease, tachyarrhythmia, osteoporosis; (6) Concurrent malignancy; (7) Pregnant, lactating, or planning pregnancy within 12 months; (8) Severe COPD, hepatic or renal insufficiency, uncontrolled hypertension or diabetes, or any condition that could bias outcome assessment; (9) Chronic use of psychotropics, systemic glucocorticoids, amiodarone, chemotherapy, iron, thionamides, PPIs, or other drugs interfering with thyroid hormone; (10) Previous exposure to levothyroxine, desiccated thyroid, or T3 preparations; (11) Participation in another clinical trial that could interfere with this study.

Stage 1 prospectively enrolls patients who underwent initial DTC surgery at participating sites, were classified as intermediate or high recurrence risk, subsequently failed to achieve target TSH suppression on L-T4 monotherapy while having FT4 ≥ 90 % of the normal upper limit. The TSH-suppressive regimen is switched to DTE + L-T4, creating a before-and-after self-control study. Stage 2 screens patients scheduled for initial DTC surgery; eligible participants are randomized 1:1 to experimental arm (DTE + L-T4 combination) and control arm (L-T4). The stage 2 target sample size is 446 patients (223 per arm).

Data collection includes: (1) Baseline characteristics: sex, age, BMI, comorbidities (cardiovascular, endocrine, thyroid, etc.), prior neck irradiation; (2) Pre-operative imaging/labs: laryngoscopy, thyroid US, TSH, FT3, FT4, TT3, TT4; (3) Operative data: procedure type, date, extent of lymph-node dissection; (4) Symptom/QoL: ThyPRO-39 questionnaire; (5) hair-pull test for alopecia; (6) Genetics: DIO2 polymorphism; (6) Post-operative plan: initial TSH-suppression dose, first dosing date, RAI administration; (7) Pathology: histology, TNM stage, molecular results, ATA risk stratum. Follow-up at 1, 3, 6, 9, 12 months post-op and 1 month after every dose adjustment: (1) Vital signs, weight, TSH-suppression details; (2) Imaging/labs: thyroid US, TSH, FT3, FT4, TT3, TT4, Tg, TgAb, calcium, 25-OH-VD; (3) Adverse events: cardiovascular, musculoskeletal, neurologic, GI, allergic reactions; (4) ThyPRO-39 and hair-pull test repeated at each visit.

Inclusion Criteria:

• Stage 1 eligibility: (1) Age ≥ 18 years；(2) Histologically proven DTC after initial surgery (± ¹³¹I)；(3) ATA recurrence risk intermediate (low-intermediate or high-intermediate) or high；(4) On L-T4 monotherapy: TSH above target despite FT4 ≥ 90 % of normal upper limit；(5) Signed informed consent and agreement to follow-up. Stage 2 eligibility: (1) Age 18-70 years (inclusive)；(2) Histologically proven DTC undergoing first-time surgery；(3) ATA recurrence risk intermediate (low-intermediate or high-intermediate) or high；(4) Signed informed consent and agreement to post-operative follow-up.

Exclusion Criteria:

• (1) Non-DTC histology; (2) Hyperthyroidism, toxic adenoma, or toxic nodular goiter; (3) Malabsorptive GI disorders (e.g., H.-pylori gastritis, atrophic gastritis, celiac disease) or history of gastric bypass; (4) Hypersensitivity to levothyroxine or desiccated thyroid; (5) Angina, coronary artery disease, tachyarrhythmia, osteoporosis; (6) Concurrent malignancy; (7) Pregnant, lactating, or planning pregnancy within 12 months; (8) Severe COPD, hepatic or renal insufficiency, uncontrolled hypertension or diabetes, or any condition that could bias outcome assessment; (9) Chronic use of psychotropics, systemic glucocorticoids, amiodarone, chemotherapy, iron, thionamides, PPIs, or other drugs interfering with thyroid hormone; (10) Previous exposure to levothyroxine, desiccated thyroid, or T3 preparations; (11) Participation in another clinical trial that could interfere with this study.