[Timely recognition and treatment of hypothyroidism in pregnant women: benefit for the child].

Three women aged 36, 34 and 33 years, with primary hypothyroidism were euthyroid whilst on levothyroxine treatment. Two women had previously received 131iodine therapy due to Graves' disease. During pregnancy the levothyroxine dose required to maintain a normal TSH level increased. The serum TSH-binding inhibiting immunoglobulins (TBII) activity was still very high (188 U/l) in one of the women, but decreased to 26 U/l in the last trimester. Foetal or neonatal thyrotoxicosis was not observed. The levothyroxine dose was decreased after delivery and none of the women experienced any adverse effects as a result of this. It is recommended that all women taking levothyroxine should have their serum TSH activity measured during the first trimester. The daily dose of levothyroxine required will usually increase by 50 micrograms during pregnancy. The determination of serum TBII activity is indicated in the case of Graves' disease; serum TBII values of > 40 U/l constitute a risk of foetal or neonatal thyrotoxicosis.