Untreated thyroid diseases in pregnancy negatively influence the health of mother and the foetus. In healthy pregnant women a serum level of TSH under 2.5 mIU/l and serum level of FT4 14-15 pmol/l are accepted in the first trimester of pregnancy. In the second and the third trimester the TSH serum level should not increase over 3 mIU/l. However, the serum level TSH > 2.5 mIU/l (with or without positive thyroid antibodies) in the first trimester already indicates a thyroid hypofunction and requires a substitution therapy. During the therapy the serum level TSH should be in the margin of 0.5-2.5 mIU/l. TSH ≥ 6 mIU/l increases the risk of foetus mortality in the first trimester of pregnancy. Mothers with thyroid hyperfunction have TSH < 0.1mIU/l, increased T4 and T3, and often also positive anti TSHr antibodies. Overt hyperthyroidism in pregnancy caused by GB disease or by toxic adenoma has to be treated with thyreostatics (Propycil). The dose of agents can be reduced or even stopped in the second trimester of pregnancy. If this is not possible thyroidectomy is indicated in the second trimester of pregnancy. However, to prevent complications in pregnancy and in foetal development all thyroid disorders have to be diagnosed before the 14th week of pregnancy.