What is the dosage adjustment for levothyroxine in pregnancy?

During pregnancy, the pharmacokinetics of levothyroxine, a synthetic thyroid hormone, are altered due to physiological changes that occur within the body. One significant change is the increase in thyroid-binding globulin (TBG) levels, which happens due to elevated estrogen levels. This increase in TBG leads to a higher demand for levothyroxine, as more of the hormone is bound and becomes inactive.

Consequently, the dosage of levothyroxine must be increased during pregnancy to ensure adequate levels of free thyroxine (the active form of the hormone) are maintained in the maternal circulation. The aim is to meet both the mother’s and the developing fetus's needs for thyroid hormone, which is critical for proper fetal growth and development, particularly in the first trimester when the fetus is entirely dependent on maternal thyroid hormones.

In summary, the necessity for an increase in levothyroxine dosage during pregnancy is primarily due to the increase in TBG levels, requiring an adjustment to maintain appropriate serum levels of free thyroxine.