Inclusion Criteria:  Boys or girls aged by 7 and 17 years old included Diagnosed with ADHD according to DSM-5 criteria with either Inattentive predominant, Hyperactive-impulsive predominant or Combined presentations. Scoring at least 28 (maximum value 54) on the ADHD-RS total score that is the sum of 18 items each rated on a 0-3 scale. Scoring at least 4 (= Moderate) on the Clinical Global Impression - Severity scale Without any clinical symptom of either hypothyroidism or hyperthyroidism Who signed an informed consent form Whose at least of parents (or default a legal representative) signed an informed consent form  Exclusion Criteria:  Known or concomitant diagnosis of Autism Spectrum Disorder according to DSM-5 criteria Known or concomitant diagnosis of Schizophrenia or Psychotic Disorder according to DSM-5 criteria Known or concomitant diagnosis of any thyroid disorder (hypothyroidism or hyperthyroidism) whatever the cause might be Known or concomitant diagnosis of any genetic condition affecting the thyroid function (e.g. Down syndrome)  Intake in the last month of a psychotropic drug for:  ADHD: methylphenidate, atomoxetine, dexamphetamine, lisdexamphetamine, guanfacine or clonidine.  Any psychiatric disorder (antipsychotic, anxiolytics, antidepressant, etc.)  Intake in the last month of a molecule affecting the thyroid function:  Any compound containing thyroid hormones Anti-thyroid drugs: carbimazole, thiamazole, propylthiouracil and perchlorate Any compound containing iodine Significant application on the skin of iodized products in the last six months Injection of water-soluble iodine contrast agents in the last 2 months or fat-soluble in the last 6 months