Starting from the diagnostic difficulties when hyperthyroidism is associated with pregnancy, PBI, total T4 and free thyroxin, triiodothyronine (T3), rT3, TBG, TSH and LATS were assayed in a group of euthyroid pregnant women (136) more or less equally distributed between month 2 and 9 of pregnancy; the same tests were applied to 14 pregnant hyperthyroid women (month 2-3), the latter also undergiving a test for urinary elimination of catecholamines. The results were referred to the normal values of techniques employed. The work presents the alterations in the thyroid status parameters when pregnancy is associated with hyperthyroidism, as against normal pregnancy, pointing out the need for performing a complex set of assays "in vitro" in order to establish correctly the diagnosis of hyperthyroidism in pregnancy and apply the right therapy at the right moment. Increased T3 and free T4 levels in hyperthyroidism referred to an increased TBG level in euthyroid pregnancy are considered as discriminators.