Management of Hypothyroidism in Pregnancy
Pregnant women with hypothyroidism require immediate levothyroxine treatment with aggressive dose titration to maintain TSH <2.5 mIU/L in the first trimester, as untreated maternal hypothyroidism significantly increases risks of miscarriage, preeclampsia, preterm birth, and permanent neurocognitive impairment in offspring. 1
Immediate Action Upon Pregnancy Confirmation
Increase levothyroxine dose by 25-50% immediately when pregnancy is confirmed—do not wait for TSH results, as most women will require this increase and first-trimester hypothyroidism specifically causes cognitive impairment in children. 1
For women already on levothyroxine who are inadequately treated, double the dose on at least three days per week to rapidly achieve euthyroidism. 2
For newly diagnosed overt hypothyroidism in pregnancy, start levothyroxine at 100-150 mcg daily to quickly restore normal thyroid function. 2
The FDA label confirms that levothyroxine should not be discontinued during pregnancy and hypothyroidism diagnosed during pregnancy should be promptly treated, as maternal hypothyroidism is associated with spontaneous abortion, gestational hypertension, pre-eclampsia, stillbirth, premature delivery, and adverse effects on fetal neurocognitive development. 3
Pre-Pregnancy Optimization (Ideal Scenario)
Target TSH <1.2 mIU/L before conception if possible, as 50% of women with pre-conception TSH of 1.2-2.4 mIU/L required dose increases during pregnancy, compared to only 17.2% when TSH was <1.2 mIU/L. 4
Confirm adequate treatment before attempting conception, as women adequately treated before pregnancy have no increased risk of perinatal morbidity. 1
Educate patients that levothyroxine requirements will increase by 25-50% during pregnancy. 1
Monitoring Protocol Throughout Pregnancy
Check TSH every 4 weeks during the first half of pregnancy, then at least once between 26-32 weeks gestation. 1
Target TSH <2.5 mIU/L specifically in the first trimester, then use trimester-specific reference ranges for subsequent monitoring. 1, 2
Maintain free T4 in the high-normal range throughout pregnancy. 1
Adjust levothyroxine by 12.5-25 mcg increments based on TSH results. 1
Critical Safety Considerations
Avoid overtreatment: TSH <0.10 mIU/L during pregnancy increases the odds of preterm delivery (adjusted OR 2.14), so aggressive treatment should not result in suppressed TSH. 5
First-trimester hypothyroidism specifically causes cognitive impairment in children—fetal harm occurs before maternal symptoms appear. 1
Clinical and subclinical hypothyroidism increase risk of preterm birth, low birth weight, placental abruption, and fetal death. 1
Meta-analyses confirm that levothyroxine reduces the risk of adverse pregnancy outcomes in overt hypothyroidism. 6
Postpartum Management
Return to pre-pregnancy levothyroxine dose immediately after delivery, as postpartum TSH levels are similar to preconception values. 3
Levothyroxine is safe during breastfeeding and may normalize milk production in hypothyroid lactating mothers with low milk supply. 3
Common Pitfalls to Avoid
Do not wait for repeat testing before initiating treatment—the 17.8% of women on thyroid replacement who did not have TSH measured during pregnancy represents a critical gap in care. 5
Do not target TSH >2.5 mIU/L in the first trimester, as even subclinical hypothyroidism associates with adverse pregnancy outcomes. 1
Do not wait for symptoms to develop before checking TSH, as fetal harm occurs before maternal symptoms appear. 1
Avoid the misconception that a "normal range" TSH is adequate—pregnancy-specific targets are more stringent. 6