Increased Levothyroxine Requirements in Early Pregnancy

Hispanic woman reading pregnancy test
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During pregnancy, a woman's thyroid gland needs to expand its hormone production significantly, in order to meet the needs of both mother and developing baby.

According to the 2011 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and Postpartum, some of the changes that take place include the following:

  • In women who are not iodine-deficient, the thyroid typically grows by as much as 10 percent in size during pregnancy.
  • In women who are iodine-deficient, the gland typically increases from 20 to 40 percent in size during pregnancy.
  • Typically, a woman's production of thyroxine (T4) and triiodothyronine (T3) increases by 50 percent during pregnancy.
  • A pregnant woman typically has a 50 percent increase in her daily iodine requirement.

According to a study reported on in the Journal of Clinical Endocrinology and Metabolism, when a woman being treated for hypothyroidism becomes pregnant, she should immediately increase her levothyroxine dosage by two tablets per week, in order to maintain her thyroid function in the normal ("euthyroid") range.

Study Results

According to the study, the addition of two additional dosages per week can significantly reduce the risk of hypothyroidism in the mother and meet the demands that early pregnancy puts on the mother's thyroid function. Failing to treat maternal hypothyroidism in early pregnancy is associated with an increased risk of miscarriage, and if the pregnancy continues, also increases the risk of stillbirth, premature birth, and developmental and cognitive delays and deficiencies in the children after birth.

In this study, the women who were hypothyroid confirmed their pregnancy and began increasing their dosage of levothyroxine medication at around 5 1/2 weeks pregnant—or around 40 days post-conception. Interestingly, at that early point, 27 percent of the study participants already had a TSH level elevated above 5.0, which is indicative of hypothyroidism and is a level associated with more negative outcomes for the pregnancy and the baby's health and cognitive development, if born.

A subset of the women did end up getting overmedicated and were taking somewhat too much thyroid medication and required further dosage adjustment. The researchers suggested that those who are more likely to end up slightly overdosed include:

  • women without a thyroid gland
  • women whose TSH level prior to pregnancy was less than 1.5
  • women who are taking at least 100 mcg/day of levothyroxine prior to pregnancy.

The study emphasized the importance of early pregnancy detection and action on the part of the patients, writing: "In clinical practice, women do not typically seek obstetrical care before 8 to 12 weeks gestation. Thus, patients themselves must understand the importance of initial (levothyroxine) adjustment immediately upon a missed menstrual cycle and a positive home pregnancy test."

According to the researchers, increasing the levothyroxine dosage by approximately 30%—two extra dosages weekly—as soon as pregnancy is confirmed "significantly reduces the risk of maternal hypothyroidism throughout the first trimester. Monitoring thyroid function approximately once monthly is required through mid-pregnancy because a minority of patients may require subsequent L-T4 dose modifications to maintain appropriate TSH concentrations."

Studies have shown that the majority of women on thyroid hormone replacement medication require as much as a 50 percent increase in dosage during the first trimester of pregnancy when the fetus relies on the mother's own production—or medication—for crucial thyroid hormone. Thyroid hormone is essential to neurological development of a fetus, particularly during the first trimester. After the first trimester, the baby's thyroid becomes capable of producing thyroid hormone, and reliance on the mother's thyroid hormone supply reduces.

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Article Sources
  • Leila Yassa, Ellen Marqusee, Rachael Fawcett, and Erik K. Alexander. "Thyroid Hormone Early Adjustment in Pregnancy (The THERAPY) Trial." Journal of Clinical Endocrinology & Metabolism. Published May 12, 2010.