Animal studies have not been reported. Thyroid hormones do not readily cross the placenta. Clinical experience to date does not indicate any adverse effects on the fetus when thyroid hormones are administered to pregnant women. There are no controlled data in human pregnancy.

AU TGA pregnancy category A: Drugs which have been taken by a large number of pregnant women and women of childbearing age without any proven increase in the frequency of malformations or other direct or indirect harmful effects on the fetus having been observed.

US FDA pregnancy category A: Adequate and well-controlled studies have failed to demonstrate a risk to the fetus in the first trimester of pregnancy (and there is no evidence of risk in later trimesters).

Use is considered acceptable.

AU TGA pregnancy category: A
US FDA pregnancy category: A

Comments:
-While clinical experience to date does not provide any evidence of fetal harm, some authorities recommend weighing the possible risk of fetal congenital abnormalities against the risk of untreated maternal hypothyroidism.

See references

Caution is recommended.

Excreted into human milk: Yes

Comments:
-May interfere with neonatal screening programmes.
-Liothyronine (T3) is a normal component of human milk; if replacement doses are required by the mother, it is not necessarily a reason to discontinue breastfeeding.
-Dose requirements may be increased in the postpartum period compared to pre-pregnancy requirements in women with Hashimoto's thyroiditis.

Due to lack of information on the exogenous use of this drug during breastfeeding, an alternate drug may be preferred.

See references