Thyroid in Pregnancy and Breastfeeding
Fetal Risk Summary
Thyroid contains the two thyroid hormones levothyroxine (T4) and liothyronine (T3) plus other materials peculiar to the thyroid gland. It is used during pregnancy for the treatment of hypothyroidism. Neither T4 nor T3 crosses the placenta when physiologic serum concentrations are present in the mother (see Levothyroxine and Liothyronine). In one report, however, two patients, each of whom had produced two cretins in previous pregnancies, were given huge amounts of thyroid, up to 1600 mg/day or more (1). Both newborns were normal at birth even though one was found to be athyroid. The authors concluded that sufficient hormone was transported to the fetuses to prevent hypothyroidism.
Congenital defects have been reported with the use of thyroid but are thought to be caused by maternal hypothyroidism or other factors (see Levothyroxine and Liothyronine).
In a surveillance study of Michigan Medicaid recipients involving 229,101 completed pregnancies conducted between 1985 and 1992, 44 newborns had been exposed to thyroid during the 1st trimester (F. Rosa, personal communication, FDA, 1993). One (2.3%) major birth defect (two expected), a cardiovascular defect (0.5 expected), was observed.
Combination therapy with thyroid-antithyroid drugs was advocated at one time for the treatment of hyperthyroidism but is now considered inappropriate (see Propylthiouracil).
Breast Feeding Summary
See Levothyroxine and Liothyronine.
Carr EA Jr, Beierwaltes WH, Raman G, Dodson VN, Tanton J, Betts JS, Stambaugh RA. The effect of maternal thyroid function on fetal thyroid function and development. J Clin Endocrinol Metab 1959;19:118.