Inclusion Criteria:  Aged between 18 and 25 years; BMI 19-25 kg/m2; Current use of iodized salt at home, adequate iodine nutritional status; Volunteer to participate in and comply with the requirement of study; Signed informed consent.  Exclusion Criteria:  1. Having history of thyroid diseases or thyroid dysfunction; 2. Inadequate iodine status (defined as mUIC <100 µg/L or >300 µg/L and assessed during screening by 3-days 24-h urine specimens) 3. Any metabolic, gastrointestinal or chronic disease or chronic use of medications or drug abuse 4. Use of iodine containing supplements within 1 month prior to study start or exposure to iodine-containing X-ray/ computed tomography contrast agent or often use of iodine-containing disinfectants 5. Cannot comply with the requirement of study or cannot delivered the excremental specimens on schedule