Inclusion Criteria:  Healthy children from 1 to 24 months  Exclusion Criteria:  recent illness, hospitalization, medication and maternal history of thyroid-related diseases, recent abnormal total leucocyte or neutrophil count in the medical record or positive microbial cultures, history of any diagnosed disease or infection, history of hospitalization during the previous four weeks, congenital hypothyroidism or transient congenital hypothyroidism, history of maternal thyroid illness, or any medications with potential influences on the thyroid function, such as amiodarone, anti-epileptic drugs, glucocorticoids