Levothyroxine is a synthetic form of the hormone thyroxine (T4), which the thyroid gland naturally produces to regulate metabolism and support bodily functions. For individuals with hypothyroidism, where the thyroid does not produce enough T4, levothyroxine serves as replacement therapy. Taking this medication is the standard and safe treatment during pregnancy, as it restores a necessary hormone. Maintaining adequate thyroid hormone levels is necessary for both the mother’s health and the development of the fetus throughout gestation.
The Essential Need for Thyroid Hormone Replacement
The body’s need for thyroid hormone significantly increases during pregnancy, sometimes requiring production to rise by 50%. For women with an underactive thyroid, levothyroxine replacement is necessary to meet this heightened demand. Untreated or undertreated maternal hypothyroidism poses serious risks to both the mother and the developing baby.
The fetus is entirely dependent on the mother’s thyroid hormone supply, especially during the first trimester, before its own thyroid gland begins functioning around 12 weeks. Thyroid hormone plays a direct role in the normal development of the fetal brain and nervous system. Deprivation of maternal T4 during this period can have irreversible effects on neurodevelopment, linked to lower IQ scores and impaired psychomotor development in children.
For the mother, inadequate thyroid hormone levels raise the risk of several pregnancy complications. These include preeclampsia (dangerously high blood pressure), placental abnormalities, anemia, preterm delivery, and miscarriage. Levothyroxine therapy mitigates these complications by maintaining hormonal balance.
Adjusting Levothyroxine Dosage During Pregnancy
The physiological changes of pregnancy create a need for a higher levothyroxine dose in approximately 50% to 85% of women. This necessity arises from hormonal changes, such as increased estrogen and the resulting rise in thyroxine-binding globulin, which binds to T4 and makes less of it active. The increased volume of blood plasma also contributes to a greater overall requirement for the hormone.
Dosage requirements can begin to increase as early as the fifth week of gestation. Some experts recommend that women increase their dose by about 20% to 30%—or by adding two extra tablets per week—immediately upon confirming the pregnancy, even before blood tests are drawn. The mean increase in levothyroxine required can be as high as 47% during the first half of pregnancy and tends to plateau by the 16th week.
Frequent monitoring of the thyroid-stimulating hormone (TSH) level is mandatory throughout gestation. Testing should be performed every four to six weeks, or more frequently following any dosage change, to ensure the TSH remains within the specific target range. The goal TSH level is much tighter than for non-pregnant adults, typically aiming for a TSH below 2.5 mIU/L during the first trimester. In the second and third trimesters, the goal TSH often targets a level below 3.0 mIU/L.
Consistency in taking the medication is important. Levothyroxine should be taken once daily on an empty stomach, generally 30 to 60 minutes before breakfast. It is important to separate the medication from prenatal vitamins, calcium, and iron supplements by at least four hours, as these can interfere with absorption.
Postpartum Management and Breastfeeding Safety
The need for an increased thyroid hormone dose drops rapidly once the pregnancy ends. The high demands created by the placenta and elevated hormones quickly subside after delivery, necessitating an immediate dose reduction to prevent the mother from becoming hyperthyroid.
Most women are advised to return their levothyroxine dosage to their pre-pregnancy level on the day they give birth. Women with Hashimoto’s thyroiditis, however, may initially require a slightly higher dose. A follow-up thyroid function test is typically scheduled around six to eight weeks postpartum to determine the long-term maintenance dose.
Levothyroxine is entirely safe for use while breastfeeding and should be continued as prescribed. It passes into breast milk in clinically insignificant amounts that are too low to influence the infant’s thyroid status. Treating hypothyroidism during lactation may help normalize milk production in mothers experiencing low supply due to the deficiency. Clinicians may also monitor for postpartum thyroiditis, a separate condition causing thyroid function fluctuations following delivery.