Levothyroxine Dosage Requirement During Pregnancy in Well-Controlled Hypothyroid Women: A Longitudinal Study

BACKGROUND: Untreated maternal hypothyroidism can have adverse effects on both the mother and fetus, but it can potentially be prevented by adequate levothyroxine replacement. This study was conducted to determine what percentage of hypothyroid pregnant women who were taking levothyroxine needed to adjust their medication dosage, and when and how much it should be increased. METHODS: In this longitudinal study, 81 well-controlled hypothyroid women (TSH≤ 2.5 mIU/L) were monitored throughout pregnancy. Thyroid function tests were performed before conception, after the first missed menstrual period, in the second and third trimesters of pregnancy and one month after delivery. Levothyroxine dosage was adjusted according to TSH levels measured. RESULTS: Of the 81 pregnancies studied, the pregnancy outcomes were 74 full-term births, six abortions and one pre-term birth. The levothyroxine dosage needed to be increased in 84% (CI95%= 74-90) of the pregnancies (OR=5.2, CI95%= 2.9-9.4). Most levothyroxine dose adjustments were made in the first trimester of gestation. The levothyroxine requirement increased 50% (CI95%= 41-59) in the first trimester, 55% (CI95%= 45-64) in the second trimester and 62% (CI95%= 52-72) in the third trimester. Levothyroxine dosage was decreased for 6 cases (7.4%), and no adjustment was made for 7 women (8.6%). CONCLUSIONS: Increases in levothyroxine dosage administered in pregnancy appear to be indispensible in the majority of patients with well-controlled hypothyroidism, especially in the first trimester. However, this change was not universal and levothyroxine dosage decreased in a few cases and remained unchanged in others.