Subclinical hypothyroidism (SCH) affects around 2% of pregnant women and 2% of children. The diagnosis is based on elevated thyroid-stimulating hormone (TS H) concentration in the presence of thyroid hormones concentrations within the population reference ranges, while clinical manifestation is very subtle. The serum thyroid-stimulating hormone (TS H) upper

reference range is 2.5 mIU /l in the first, 3.0 mIU /l in the second, and 3.5 mIU /l in the third trimester. In children TS H levels varies according to age. SCH in pregnancy is unfavourable, because it may be associated with miscarriage, gestational diabetes, preterm delivery and other complications. The influence on children’s neuropsychological development remains unclear. It is believed that SCH in pregnancy should be treated, the aim is TS H level below trimester-specific reference ranges. Hormonal substitution in children with SCH is more controversial, because elevated TS H level often decreases spontaneously and the benefits of treatment are unclear. It is recommended to treat children with TS H over 10 mIU /l that does not normalize, with goiter, symptoms of hypothyroidism or chronic comorbidities associated with SCH, such as Down syndrome. L-thyroxine is the first-line drug. In Poland every newborn is screened for hypothyroidism after birth. Such test is not obligatory during pregnancy, although some endocrine societies recommend it and many physicians order TS H measurement during pregnancy.