A 26-year-old woman calls the office with a question about management of hypothyroidism during pregnancy. The patient is not pregnant but wishes to become pregnant soon. She has a history of hypothyroidism for several years and has been on a stable dose of levothyroxine. TSH checked 3 months ago was 2.0 µU/mL. The patient currently has mild fatigue but is otherwise asymptomatic. Family history is notable for primary hypothyroidism in her mother, who is doing well on a regimen of liothyronine (T₃) and levothyroxine (T₄). Which of the following is the most appropriate recommendation for this patient?

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11ec25c8_d21e_bf3e_bc81_81774580aae9_MD0003_00 During pregnancy, estrogen induces an increase in serum thyroxine-binding globulin levels, requiring an increase in the amount of thyroid hormone needed to saturate the binding sites. Thyroxine production is also increased due to the stimulatory effects of hCG on TSH receptors (as hCG and TSH have a similar alpha subunit). These changes lead to an increase in total thyroid hormone levels, with a minimal increase in free hormone levels and a decrease in TSH. However, patients with pre-existing hypothyroidism are unable to increase thyroxine production appropriately and are at risk for a worsening hypothyroid state and adverse fetal and maternal effects (eg, gestational hypertension, preeclampsia, premature delivery, postpartum hemorrhage). Individuals such as this patient on a stable dose of thyroid replacement should have their dose increased by approximately 30% at the time the pregnancy is detected (Choices A and B). The dose should be adjusted subsequently (typically in 4-week increments) based on TSH using pregnancy-specific norms. Total T₄ should also be followed and maintained within pregnancy-specific norms. (Choice C) This patient has been on a stable dose of levothyroxine and has a TSH in the low-normal range (<2.5 µU/mL). She does not require dosing adjustments at this time. (Choice E) T₃ is the active form of thyroid hormone and is produced predominantly by conversion from T₄ in peripheral tissues. However, T₃ has a short half-life, and supplementation with liothyronine (oral T₃) is associated with wide fluctuations in blood levels. T₃ supplementation is not recommended for most patients with primary hypothyroidism, and those who are taking T₃ should be changed to levothyroxine alone if considering pregnancy. Educational objective: Levothyroxine requirements increase during pregnancy. Patients with hypothyroidism should increase their levothyroxine dose at the time pregnancy is detected, with subsequent dose adjustments based on TSH and total T₄ using pregnancy-specific norms. References: Management of thyroid dysfunction during pregnancy and postpartum: an Endocrine Society clinical practice guideline. (http://www.ncbi.nlm.nih.gov/pubmed/22869843)

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