A 23-year-old woman comes to the office for follow-up. The patient has a 5-year history of hypothyroidism and has been on a stable dose of levothyroxine for the past 3 years. She is now 6 weeks pregnant with her first child.
Physical examination findings are noncontributory.
Results of laboratory studies 1 month ago showed a serum thyroid-stimulating hormone (TSH) level of 2.9 µU/mL (2.9 mU/L) and a free thyroxine level of 1.4 ng/dL (18.1 pmol/L).
Which of the following is the most appropriate management?
Answer and Critique (Correct Answer: C)
Educational Objective:Manage hypothyroidism during pregnancy.
- Because the fetus is dependent on maternal thyroid hormone during the first trimester, guidelines for the treatment of hypothyroidism during pregnancy recommend that the total thyroxine (T4) level be kept stable at 1.5 times the normal range and the thyroid-stimulating hormone level be kept in the lower range of normal.
The most appropriate next step is to recheck this patient’s serum thyroid-stimulating hormone (TSH) level. Because a fetus depends on maternal thyroid hormone for the first 10 to 12 weeks of gestation, the thyroid levels of pregnant women with hypothyroidism should be carefully monitored. Recent guidelines recommend that TSH and total thyroxine (T4) levels be monitored throughout pregnancy because standard free T4 levels are not as accurate in pregnant patients. The total T4 level should be kept stable at approximately 1.5 times the normal range, and the TSH level should be kept in the lower range of normal. Because of estrogen elevation during pregnancy, thyroid-binding globulin (TBG) levels increase. However, without an increase in the dosage of levothyroxine, free T4 levels may decrease as more T4 becomes bound by TBG. After delivery, TBG levels decrease, as do thyroid hormone requirements.
Because the patient’s TSH level was already borderline high 1 month ago, it should be rechecked. If the TSH level is any higher now, an increase in the levothyroxine dosage is warranted. Pregnant patients with hypothyroidism may require an increase in their levothyroxine dosage of approximately 35% to 50% as early as the first trimester.
Although maternal iodine replacement has been successfully used in countries with prevalent iodine deficiency, its use in patients who are iodine sufficient can be associated with catastrophic results, such as a fetal goiter. Because significant iodine deficiency in the United States is rare, iodine therapy in pregnant U.S. women is not indicated.
Measurement of the free triiodothyronine (T3) level is not useful in the evaluation of hypothyroidism because T3 levels typically remain within the reference range until the point of severe hypothyroidism. This pattern is unaltered by pregnancy.
Continuing the current management is inappropriate because undertreatment of maternal hypothyroidism can have a potentially negative effect on fetal neurocognitive development.
- Abalovich M, Amino N, Barbour LA, et al. Management of thyroid dysfunction during pregnancy and postpartum: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2007;92(8 Suppl):S1-S47. [PMID:17948378] - See PubMed
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