Overt and subclinical maternal hypothyroidism is known to have serious adverse effects for both mother and fetus. Given the complex physiology of thyroid function during pregnancy, hormone assessment should be performed according to reference values for each gestational trimester and generated locally in each reference laboratory. If trimester-specific references intervals are not available in the laboratory, the following reference ranges of tirotropine are recommended: first trimester < 2,5 mUI/L; second and third trimesters < 3,0 mUI/L.