CASE 1: STABLE HYPOTHYROIDISM
A 29-year-old woman with stable primary hypothyroidism calls your office to report that she is pregnant. She has taken levothyroxine (100 mg) for the past three years, and her TSH level was 1.21 mIU/L at last measurement. She denies any symptoms of hyperthyroidism or hypothyroidism. What is your next step in her management?
Recommendation
The American Thyroid Association recommends monitoring serum TSH every four weeks during the first half of pregnancy and at least once per trimester thereafter, with frequency depending on symptoms and TSH levels.3 Most women will require higher doses of levothyroxine supplementation to maintain therapeutic TSH levels.
Prior to 18 weeks’ gestation, the fetus is dependent on maternal thyroid hormone. When pregnancy is confirmed, there is support in the literature for having the patient take two additional doses of levothyroxine per week until TSH can be tested.4 However, many endocrinology practices opt to check TSH and total T4 as soon as pregnancy is confirmed.
Since free T4 results may be unreliable during pregnancy (due to the effect of TBG), free thyroxine index (FTI) or total T4 should be monitored instead. FTI mathematically corrects free T4 for TBG levels, making it a useful marker. If total T4 is measured, it is important to remember that results will be approximately 1.5x the nonpregnancy value; thus, the reference range must be multiplied by 1.5 to calculate appropriate high and low parameters for pregnant patients.
Ideally, all women of childbearing age should be encouraged to plan pregnancy, to ensure TSH is at target prior to conception. Maintaining a euthyroid state throughout pregnancy (starting at conception) is important to decrease risk for such adverse outcomes as spontaneous abortion, placental abruption, and gestational hypertension.2 Low birth weight and respiratory distress are potential complications for newborns whose mothers have inadequately controlled hypothyroidism.
Patients should be counseled against simultaneous dosing of prenatal vitamins and levothyroxine. Prenatal vitamins contain iron, which reduces absorption of levothyroxine; therefore, it is recommended that the levothyroxine be taken four hours or more apart from prenatal vitamins.
The Endocrine Society recommends a TSH no higher than 2.5 mIU/L for hypothyroidism diagnosed prior to pregnancy.2,3 After delivery, levothyroxine doses should be reduced to prepregnancy levels, with close monitoring of TSH.2
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