This pregnant patient's levothyroxine dose should be increased to lower the serum thyroid-stimulating hormone. Maternal thyroid hormone production typically increases by 30% to 50% during pregnancy; therefore, in pregnant patients requiring levothyroxine supplementation, the replacement dose usually needs to be increased to provide adequate thyroxine (T₄) for the neurologic development of the fetus. The combination of this patient's elevated thyroid-stimulating hormone (TSH) level and low total T₄ level suggest that her levothyroxine dose should be increased. During pregnancy, the physiologic changes in thyroid hormone levels include a reduction in the serum TSH level and an increase in the serum total T₄ level. This change in the serum TSH level is partly due to the rise in the serum human chorionic gonadotropin (HCG) level; both hormones share sequence homology in their α subunit. As the serum HCG level rises with progression of pregnancy, the hormone can bind to the TSH receptors, resulting in a reduction in serum TSH levels. Consequently, the normal reference range for serum TSH during pregnancy shifts to a lower value, from 0.5 to 5.0 µU/mL (0.5-5.0 mU/L) pre-pregnancy to 0.03 to 2.5 µU/mL (0.03-2.5 mU/L) during the first trimester. Additionally, the serum total T₄ level rises 1.5-fold above the normal nonpregnant reference range. Part of this increase is due to the higher levels of estrogen associated with pregnancy, which cause an increase in serum total protein levels, including serum thyroid hormone–binding proteins. The increased thyroid hormone requirements of the fetus also contribute to this change in serum T₄ levels.

In this patient, with a serum TSH value above 2.5 µU/mL (2.5 mU/L) during the first trimester, the dose of levothyroxine needs to be increased rather than decreased or discontinued. The serum TSH level should be rechecked in 4 weeks to ensure that the dose adjustment continues to be adequate. Likewise, thyroid function tests should be repeated at least once during each trimester to ensure that additional adjustments in the levothyroxine dose are not needed.