Levothyroxine as Adjuvant to a Hypocaloric Diet for the Treatment of Obesity.

Inclusion Criteria: * Adults 25-60 years of age. * Obesity grade I-II: BMI 30.0-39.9 kg/m². * Either subclinical hypothyroidism (TSH 5-10 mIU/L with normal peripheral hormones) or euthyroid with TSH in the highest tertile of the population reference range (no known thyroid disease). * Able and willing to provide written informed consent. * Women of childbearing potential: not pregnant or breastfeeding and using a highly effective contraception method (failure rate \<1%) per CTCG guidance (e.g., hormonal methods, IUD/IUS, sterilization, or dual barrier with spermicide). Exclusion Criteria: * Diabetes mellitus (HbA1c ≥6.5%, fasting glucose ≥126 mg/dL, or 2-h OGTT ≥200 mg/dL). * Any prior thyroid disease (hyperthyroidism, overt hypothyroidism) or prior LT4 treatment. * Current or recent (≤3 months) use of levothyroxine, hypoglycaemic agents, antibiotics, or regular probiotics/prebiotics. * Active cancer or cancer within the last 5 years (except basal-cell carcinoma). * Chronic liver disease with total bilirubin ≥2.0 mg/dL or AST \>3× ULN. * Established cardiovascular disease (e.g., stroke, ischemic heart disease, peripheral artery disease). * Atrial fibrillation or any arrhythmia history. * Uncontrolled hypertension (\>160/100 mmHg) despite therapy (assessed by ABPM at screening). * Any heart failure; resting HR \>85 bpm; eGFR \<60 mL/min. * Known HIV, HBV, or HCV infection. * Acute inflammatory disease or inflammatory bowel disease. * Serious underlying disease that, in the investigator's judgment, could affect participation. * Drug/alcohol abuse, life expectancy \<12 months, inability to follow the recommended diet or attend visits. * Positive pregnancy test, pregnant, expecting pregnancy, or breastfeeding. * Hypersensitivity to any component of the investigational product. * Inability or unwillingness to provide informed consent.