Thyroid hormones play fundamental role in conception and pregnancy and are essential for normal adult health, fetus and childhood development. Many studies have shown an association between maternal thyroid diseases esp. hypothyroidism with obstetric problems and/or psychomotoric impairment in the offspring. The prevalence of undiagnosed lower thyroid function in pregnancy is present in about 4-8 % of pregnant women, and euthyroid women with thyroid autoimmunity (6-8 %) are further candidates for thyroid disorders in pregnancy. The thyroid gland needs to produce 50 % more thyroxine in pregnancy to maintain an euthyroid state to keep TSH ideally 2.5 mIU/l in the first trimester of pregnancy and TSH 3.0 mIU/l in the second and third trimester. Consequently, there is a need to start the substitution therapy as soon as diagnosis of subclinical and /or overt hypotyroidism is established, and in majority of euthyroid women with autoimmune thyroid disease there is a need to start therapy as well. Most women on levothyroxine therapy before pregnancy require an increase in dose when pregnant. As maternal thyroid disease is a quite prevalent condition and often asymptomatic, but easily diagnosed and for which an effective, safe and cheap treatment is available, endocrinological societies including ČES ČLS JEP worldwide are suggesting the need of thyroid dysfunction screening as a simple prevention attitude. Hormone determination of TSH and TPOab antibodies should be performed early during the first trimester, using trimester-specific reference values. Furthermore, adequate iodine supplementation during pregnancy is critical and if feasible it should be initiated before the woman attempts to conceive.