Management of Triiodothyronine (T3)-Predominant Graves' Disease in Pregnancy: A Case Series

Graves' disease (GD) in pregnancy requires the careful titration of antithyroid drugs and close monitoring to avoid maternal, fetal and neonatal complications. GD can occasionally be triiodothyronine (T3)-predominant, with normal or low free thyroxine (fT4) despite high free T3 (fT3) and suppressed thyroid-stimulating hormone levels. While guidelines advocate keeping the fT4 at or just above the upper limit of normal (ULN) in conventional GD in pregnancy, there is little literature on the optimal management for T3-predominant GD in pregnancy.

We describe 3 pregnancies (2 patients age 23 and 32) with T3-predominant GD in pregnancy treated with propylthiouracil titrated to aim fT3 at or slightly above ULN with good outcomes. fT4 of these patients were occasionally low or low-normal, which is controversial as the fetus is deemed dependent on maternal fT4 (not fT3) especially in the first trimester for development. However, allowing fT4 to be at or slightly above ULN would entail very high fT3 levels and severe maternal hyperthyroidism. There were no signs of fetal hypothyroidism or hyperthyroidism during the pregnancies. In all 3 pregnancies, they delivered at term via normal vaginal delivery. One neonate had mild thyrotoxicosis not requiring treatment which resolved within a few weeks, and the other 2 neonates were euthyroid. No other complications were observed.

We discuss the pathophysiology and considerations behind our approach.

Aiming for fT3 (instead of fT4) at or slightly above ULN to avoid maternal overt hyperthyroidism appears to be a reasonable approach that physicians can consider when treating T3-predominant GD in pregnancy.