Thyroxine and Levothyroxine in Pregnancy: They Are the Same Medication
Thyroxine (T4) and levothyroxine are identical medications—levothyroxine is simply the synthetic form of thyroxine, and both terms refer to the same thyroid hormone replacement used in pregnancy. 1, 2
Understanding the Terminology
- Levothyroxine is the pharmaceutical name for synthetic T4 (thyroxine), making them interchangeable terms for the same medication 3, 4
- All guidelines and clinical practice universally recommend levothyroxine (synthetic T4) as the standard treatment for hypothyroidism during pregnancy 1, 4, 5
- There is no distinction between "thyroxine" and "levothyroxine" in clinical practice—they represent the same molecule 3, 6
Why Levothyroxine Is the Standard in Pregnancy
- Levothyroxine is the only recommended thyroid hormone replacement during pregnancy because it provides stable T4 levels that can be converted to T3 as needed by maternal and fetal tissues 1, 4
- The goal is to maintain free T4 in the high-normal range using the lowest possible medication dose, with TSH monitored every trimester 1
- Untreated hypothyroidism during pregnancy causes severe adverse effects including preeclampsia, low birth weight, and impaired fetal neuropsychological development 1, 3, 5
Critical Pregnancy-Specific Dosing Requirements
- Levothyroxine requirements increase by approximately 30-50% as early as the fifth week of gestation, with the median onset at eight weeks 7
- Women with pre-existing hypothyroidism should increase their levothyroxine dose by approximately 30% as soon as pregnancy is confirmed 7
- The increased dose requirement plateaus by week 16 and remains elevated until delivery 7
- Maternal hypothyroxinemia (low T4 with normal TSH) requires treatment with levothyroxine to restore T4 levels and prevent fetal neuropsychological defects 1
Monitoring and Target Levels
- Evaluate thyroid function every trimester to adjust the levothyroxine dose as necessary 1
- The target TSH during pregnancy should be below 2.5 μIU/mL, particularly in the first trimester when fetal brain development is most vulnerable 5
- For subclinical hypothyroidism with TSH >10 mIU/L or TSH above trimester-specific reference ranges with positive thyroid autoimmunity, levothyroxine treatment is strongly recommended 1, 5
Common Pitfalls to Avoid
- Failing to increase levothyroxine dose early in pregnancy leads to inadequate treatment, which is associated with low birth weight and adverse neurodevelopmental outcomes 1, 7
- Real-world evidence shows hypothyroidism in pregnancy is often overlooked or levothyroxine is not dosed appropriately to achieve tight TSH control 5
- Approximately 25% of patients on levothyroxine are unintentionally maintained on inappropriate doses, emphasizing the need for vigilant monitoring 2