Synthroid (Levothyroxine) During Pregnancy: Dose Changes

What does the evidence say about synthroid during pregnancy?

1. Thyroid hormone needs often rise early in pregnancy: one study found the levothyroxine dose had increased by about 47% by the first trimester and then held roughly steady for the rest of pregnancy, though some people need no change at all. 1
2. Because needs can keep shifting, TSH and dosing are typically reviewed about every 4 weeks during pregnancy, more often than the standard 6 to 8 week interval used outside of pregnancy, and T3 or desiccated thyroid preparations are not recommended in pregnancy. 2
3. In a widely cited study, children born to mothers with overt hypothyroidism that wasn't on levothyroxine scored an average of 7 IQ points lower at ages 7 to 9 than children of mothers with normal thyroid function, and a large 2013 cohort study linked unmanaged overt hypothyroidism to higher rates of preeclampsia, gestational diabetes, preterm birth, and NICU admission. 3
4. Levothyroxine passes into breast milk only in small amounts, with a milk-to-serum ratio of about 0.3 for T4, and the American Thyroid Association recommends levothyroxine for hypothyroidism in mothers who are breastfeeding. 4
5. After delivery, thyroid hormone requirements generally return toward pre-pregnancy levels, and for people who needed only a low dose during pregnancy, the requirement can drop to zero for a while. 1

Is synthroid safe in each trimester?

• First trimester. Thyroid hormone needs often start rising as soon as pregnancy begins. One study found the dose had already increased by about 47% by the first trimester and held roughly steady for the rest of pregnancy. Contact your provider as soon as you know you're pregnant so your dose can be reviewed.
• Second trimester. Dose and TSH are typically rechecked about every 4 weeks through this stage, more often than the standard 6 to 8 week interval outside of pregnancy, since requirements can keep shifting.
• Third trimester. Needs usually plateau by now for most people, though monitoring continues. After delivery, the dose is typically brought back down toward your pre-pregnancy amount, and for some people on a low pregnancy dose, it can drop to zero for a while.

Levothyroxine dosing signals in pregnancy