The Magnitude of Increased Levothyroxine Requirements in Hypothyroid Pregnant Women Depends upon the Etiology of the Hypothyroidism

Background: In the United States, many women with hypothyroidism are on thyroid hormone replacement during pregnancy. The optimal management strategy for thyroid hormone dosing in hypothyroid women during pregnancy is controversial. We hypothesized that dosage requirements during pregnancy might differ depending upon the nature of the underlying hypothyroidism. Methods: We conducted a retrospective review of 45 pregnancies from 38 women whose hypothyroidism was managed during pregnancy. Thyroid function tests were obtained when pregnancy was confirmed, then every 4-8 weeks. The thyrotropin (TSH) goal was 0.4-4.1 mu U/mL (SI unit conversion: multiply TSH by 1.0 for mIU/L). Results: On average, the entire group required a cumulative increase from baseline in levothyroxine (LT4) dosage of 13% in the first trimester, 26% in the second trimester, and 26% in the third trimester (p < 0.001, p < 0.001, p < 0.001, respectively). Average baseline LT4 dose for patients with primary hypothyroidism was 92.5 +/- 32.0 mu g daily. These patients required small cumulative dose increases of 11%, 16%, and 16% from baseline in each trimester, respectively (p values 0.125, 0.016, 0.016). Average baseline LT4 dose for patients with hypothyroidism resulting from treated Graves' disease or goiter was 140.4 +/- 62.4 mu g daily. These patients required the largest cumulative increases in LT4 dosage (first trimester, 27%; second trimester, 51%; third trimester, 45%; p = 0.063, 0.063, 0.063, respectively). Average baseline LT4 dose for patients with thyroid cancer was 153.2 +/- 30.3 mu g. The cumulative LT4 dose increases for patients with thyroid cancer were 9%, 21%, and 26% in each trimester, respectively (p = 0.03, p < 0.001, p < 0.001). Conclusions: The etiology of hypothyroidism plays a pivotal role in determining the timing and magnitude of thyroid hormone adjustments during pregnancy. Patients require vigilant monitoring of thyroid function upon confirmation of conception and anticipatory adjustments to LT4 dosing based on the etiology of their hypothyroidism.