Is Levothyroxine (Synthroid) safe in pregnancy?

Common uses

Hypothyroidism

How Levothyroxine (Synthroid) works and why pregnancy changes the math

Levothyroxine (Synthroid) replaces a hormone your body should be making — in this case thyroid hormone. For someone with hypothyroidism, the medication is not adding something foreign; it is restoring a level your body and your developing baby both depend on. Untreated maternal hypothyroidism is associated with miscarriage, preterm birth, and impaired neurodevelopment.

For pregnancy the calculation is straightforward: continue the medication and expect your dose requirement to go up. Most people with hypothyroidism need a 25-50% dose increase across pregnancy because the body's thyroid hormone demand rises. Frequent TSH monitoring through pregnancy is standard care so dose adjustments stay current.

How Levothyroxine (Synthroid) risk changes by trimester

The clinical reasoning behind the verdict

Replaces thyroid hormone your body should be making. Critical for fetal development.

Dosing and what to do if symptoms keep going

Pregnancy dosing for Levothyroxine (Synthroid) generally follows standard adult guidance unless your provider has directed otherwise. Pregnancy changes how your body absorbs, distributes, and clears many medications, so doses that worked before may need adjustment as pregnancy progresses.

If symptoms are not responding to standard dosing of Levothyroxine (Synthroid), that is a conversation with your prescriber rather than a reason to escalate on your own. Pregnancy is a time when changes to medication should happen with provider involvement, both because the underlying condition may be evolving and because pregnancy-safe alternatives may be available.

Safer alternatives and how to choose between them

None — continue treatment.

The right alternative depends on what Levothyroxine (Synthroid) was being used to treat. For mild symptoms, non-medication approaches often work — saline rinses for congestion, ice for swelling, heat for muscle pain, rest for fatigue. For ongoing conditions, pregnancy-safe medications usually exist and are best identified with your provider's input.

The trap to avoid is stopping a needed medication abruptly without a replacement plan, especially for chronic conditions like hypertension, diabetes, depression, or autoimmune disease. Untreated maternal conditions usually carry pregnancy risks of their own, sometimes larger than the risks of the medication being avoided. A pregnancy-aware substitute usually beats stopping treatment.

What recent research has been saying about Levothyroxine (Synthroid)

The literature on Levothyroxine (Synthroid) in pregnancy continues to evolve as more population-level data accumulates and as researchers control more carefully for confounding factors. The pregnancy-specific evidence base for any given medication is rarely as deep as the general adult evidence base, so cautious clinical interpretation and individualized provider conversation remain the right approach as guidance updates.

Sources and further reading

ACOG Thyroid Pregnancy 2024