Thyroid Laboratory Testing and Management in Women on Thyroid Replacement Prior to Pregnancy and Associated Pregnancy Outcomes.

BACKGROUND Women with hypothyroidism prior to pregnancy often require an increase in their levothyroxine dosage to maintain a euthyroid state during pregnancy. The objectives of this study were to investigate: 1) the frequency and distribution of thyrotropin (TSH) testing and levothyroxine dosage adjustment by gestational age, 2) the magnitude of levothyroxine increase by underlying etiology of hypothyroidism and 3) the relationship between overtreatment or undertreatment during pregnancy with adverse pregnancy outcomes among women using thyroid replacement prior to pregnancy. METHODS A retrospective cohort study of pregnancies in women on thyroid replacement prior to pregnancy in Alberta, Canada was performed. Women using thyroid replacement anytime during the two years prior to pregnancy who delivered between October 2014 and September 2017 were included. Delivery records, physician billing, laboratory and pharmacy administrative data were linked. Outcomes included characteristics of TSH testing, levothyroxine dosing, and pregnancy outcomes. The frequency and gestational timing of TSH testing and levothyroxine adjustments were calculated. Multiple logistic regression was used to test whether pregnancies with TSH 10.00 mIU/L (undertreatment) compared to control pregnancies (TSH 0.10-4.00 mIU/L) were associated with adverse pregnancy and neonatal outcomes. RESULTS Of the 10,680 deliveries, 8774 (82.2%) underwent TSH testing at least once during pregnancy, at a median gestational age of 6 weeks. An adjustment of levothyroxine dosage was made for 4,321 (43.7%) during pregnancy. TSH in pregnancy below 0.10 mIU/L increased the odds of preterm delivery when compared to control pregnancies (adjusted OR, 2.14 [95% confidence interval 1.51, 2.78]). TSH>10.00 mIU/L during pregnancy was not associated with any adverse pregnancy or neonatal outcomes in the multivariable analysis. CONCLUSIONS Although most women on thyroid replacement prior to conception had TSH measured at some point during pregnancy, it is concerning that 17.8% did not. Levothyroxine overtreatment in pregnancy was associated with preterm delivery. These findings suggest that clinicians should be careful to avoid overtreatment with levothyroxine in pregnancy.