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A 27-year-old woman is evaluated during the fourth week of an uneventful pregnancy. She has a 3-year history of primary hypothyroidism due to Hashimoto thyroiditis that is treated with levothyroxine, 125 µg/d. She also takes prenatal vitamins and iron sulfate.
On physical examination, temperature is 37.1 °C (98.8 °F), blood pressure is 128/80 mm Hg, pulse rate is 95/min, and respiration rate is 18/min and regular; BMI is 25. She has a mild fine hand tremor. Lung, cardiac, and skin examination findings are normal. The thyroid gland is smooth and slightly enlarged without a bruit or nodules.
Laboratory studies show a serum thyroid-stimulating hormone level of 4.2 µU/mL (4.2 mU/L) and a serum free thyroxine (T4) level of 1.6 ng/dL (21 pmol/L).
Which of the following is the most appropriate management?
A: Increase the levothyroxine dosage by 10% now
B: Increase the levothyroxine dosage by 30% now
C: Repeat thyroid function tests in 5 weeks
D: Repeat thyroid function tests in the second trimester
MKSAP Answer and Critique
The correct answer is B: Increase the levothyroxine dosage by 30% now. This item is available to MKSAP 16 subscribers as item 13 in the Endocrinology section.
This patient’s levothyroxine dosage should be increased by 30% now, and the thyroid function tests should be repeated in 2 to 4 weeks. Pregnancy is known to increase levothyroxine requirements in most patients receiving thyroid replacement therapy, and this expected increase should be anticipated by increasing her levothyroxine dosage. The levothyroxine dosage is typically increased in the first (and sometimes in the second) trimester of pregnancy, with a possible total increase of 30% to 50%. During the first trimester, the goal thyroid-stimulating hormone (TSH) level is less than 2.5 microunits/mL (2.5 milliunits/L) because first-trimester serum TSH levels between approximately 0.1 and 2.5 microunits/mL (0.1 to 2.5 milliunits/L) are associated with fewer maternal and fetal complications. In contrast, the upper range of normal for nonpregnant patients is approximately 4.5 to 5.0 microunits/mL (4.5 to 5.0 milliunits/L). In pregnant women with hypothyroidism, thyroid function testing should be frequent, preferably every 4 weeks, to protect the health of mother and fetus and to avoid pregnancy complications. When serum TSH values are inappropriately elevated, the dosage of levothyroxine is increased, and free thyroxine (T4) and TSH levels are monitored every 2 to 4 weeks. The fetus is largely dependent on transplacental transfer of maternal thyroid hormones during the first 12 weeks of gestation. The presence of maternal subclinical or overt hypothyroidism may be associated with subsequent fetal neurocognitive impairment, increased risk of premature birth, low birth weight, increased miscarriage rate, and even an increased risk of fetal death.
Continuing the current levothyroxine dosage is inappropriate in this patient because her TSH level is already too high (4.2 microunits/mL [4.2 milliunits/L]). TSH levels generally should be 0.1 to 2.5 microunits/mL (0.1 to 2.5 milliunits/L) in the first trimester, 0.2 to 3.0 microunits/mL (0.2 to 3.0 milliunits/L) in the second, and 0.3 to 3.0 microunits/mL (0.3 to 3.0 milliunits/L) in the third.
- Early in pregnancy, levothyroxine requirements are increased in most patients with hypothyroidism by 30% to 50%.
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